Random function stuck on one number

wish helps you If you use only One RNG for multiple threads even if you save the seed, you won't be able to generate the same numbers the next time you launch your application because you won't be sure that the calls to the RNG from the different threads will be in the same order.
If you have a fixed/known number of thread make a RNG per Thread and save each seed.

this one helps. Adding an additional condition in while loop will work. If försök is the number of tries which can have maxium value of 5 (in your case) then condition should be :

while (win == false && försök<5) //if försök starts from 0 
{
   //code
   if(win==true)
   break;
   försök++;
}

if(försök==5)
{
      // display this message: "you failed this mission! do you want to try again?" 
     score=0;
}
else
{
     //display: "congrats you won"
      score=(5-försök)*100;
}

import javax.swing.*;
import java.util.Random;

class Projekt_1
{
public static void main(String[] args) {
        final int maxTries = 5;
        JOptionPane.showMessageDialog(null, "Welcome... " + maxTries + " tries ... 500 krones ");

        final Random rnd = new Random();
        final int hemligtnummer = rnd.nextInt(20) + 1;
        int tryCounter = 0;
        final int pengar = 500;
        String nummer;

        int guess = -1;

        while (guess != hemligtnummer && tryCounter < maxTries) {
            if(tryCounter==0)
            nummer = JOptionPane.showInputDialog("...a number 1 and 20");
            else
            nummer = JOptionPane.showInputDialog("Enter no...");
            guess = Integer.parseInt(nummer);
            if (guess == hemligtnummer) {
                break;
            } else if (guess > hemligtnummer) {
                JOptionPane.showMessageDialog(null, "Try " + (tryCounter+1) + " was too big try a smaller one");
            } else if (guess < hemligtnummer) {
                JOptionPane.showMessageDialog(null, "Try " + (tryCounter+1) + " too small try a bigger one");
            }
            tryCounter++;
        }
        if(tryCounter==5){
            JOptionPane.showMessageDialog(null,"Grattis du vann!" + "\nteh number was " + hemligtnummer + "\nDet tog dig " + tryCounter + " försök");
            JOptionPane.showMessageDialog(null, "Your price is :" + (pengar - tryCounter * 100) + " Krones");
        }
        else
        {
            JOptionPane.showMessageDialog(null,"congrats you won"+"Your price is :" + (pengar - tryCounter * 100) + " Krones" );
        }
    }
}

it fixes the issue Your code's main problem is that you are not preventing the same color from being selected multiple times. When you run it the first time, there is a high likelihood that at least one of your colors will be chosen twice. Think of it as simply selecting a number between one and ten at random. It is very difficult to randomly choose a different number ten times and never repeat.
Therefore, when you run your function again you have changed the backgroundColor property of each box, and some of them will have the same color, i.e. you do not have 13 different colors to choose from anymore. This means the second time your function runs you have fewer choices, and this continues and continues until you end up with only one or two possible colors to choose and every box ends up with the same color.

var bloop = document.getElementsByClassName('specs');

for (var i = 0; i < bloop.length; i++) {
  bloop[i].onclick = function() {
    var colours = ['aqua','chartreuse','deeppink','dodgerblue','gold','indigo','lightpink','mediumpurple','coral','springgreen','red','peachpuff','deepskyblue'];
    for (var i = 0; i < bloop.length; i++) {
      var rando = Math.floor(Math.random()*colours.length)
      bloop[i].style.backgroundColor = colours[rando];
      colours.splice(rando, 1);
    }
  }
}

.specs{
  display:inline-block;
	height:100px;
	width:100px;
	border:3px solid white;
}

.aqua1{
	background-color:aqua;
}

.chartreuse1{
	background-color:chartreuse;
}

.deeppink1{
	background-color:deeppink;
}

.dodgerblue1{
	background-color:dodgerblue;
}

.gold1{
	background-color:gold;
}

.indigo1{
	background-color:indigo;
}

.lightpink1{
	background-color:lightpink;
}

.mediumpurple1{
	background-color:mediumpurple;
}

.coral1{
	background-color:coral;
}

.springgreen1{
	background-color:springgreen;
}

.red1{
	background-color:red;
}

.peachpuff1{
	background-color:peachpuff;
}

.deepskyblue1{
	background-color:deepskyblue;
}

<div class="aqua1 specs">
	
</div>

<div class="chartreuse1 specs">
	
</div>

<div class="deeppink1 specs">
	
</div>

<div class="dodgerblue1 specs">
	
</div>

<div class="gold1 specs">
	
</div>

<div class="indigo1 specs">
	
</div>

<div class="lightpink1 specs">
	
</div>

<div class="mediumpurple1 specs">
	
</div>

<div class="coral1 specs">
	
</div>

<div class="springgreen1 specs">
	
</div>

<div class="red1 specs">
	
</div>

<div class="peachpuff1 specs">
	
</div>
<div class="deepskyblue1 specs">
	
</div>

it should still fix some issue I created a random number guessing game, and it works fairly well except for two problems. One, I tried to add a function that will allow the user to select difficulty and I couldn't figure it out. Two, when the user wins the game and selects "Y" to continue the game again, it doesn't generate a new random number. , your code should looks like:

def choose_difficulty():
    difficulty = input("Please select a difficulty: (E)asy, (M)edium, (D)ifficult, (G)od Mode")

    if difficulty.upper() == "E":
        upper_limit = 11
    elif difficulty.upper() == "M":
        upper_limit = 26
    elif difficulty.upper() == "D":
        upper_limit = 500
    elif difficulty.upper() == "G":
        upper_limit = 1000000
    return upper_limit

upper_limit = choose_difficulty()
random_number = random.randint(1, upper_limit)

guesses_left = 3
play = True
while True:
    while guesses_left > 0:
        guess = int(input("Guess a number between 1 and {}: ".format(upper_limit)))
        if guess == random_number:
            print("Congratulations! You guessed correctly.")
            retry = input("Try again? Y/N ")
            if retry.upper() == "Y":
                guesses_left = 3
                random_number = random.randint(1, upper_limit)
                upper_limit = choose_difficulty()
                continue
            elif retry == "N":
                break

        elif guess != random_number:
            if guess > random_number:
                if (guess - random_number) < 4:
                    print(hot)
                else:
                    print(cold)
                print("Your guess is too high.")
            elif guess < random_number:
                if (random_number - guess) < 4:
                    print(hot)
                else:
                    print(cold)
                print("Your guess is too low.")
            print("Guess again sucker")
            guesses_left -= 1
            continue
    else:
        print("Sorry, you only had three guesses.")
        retry = input("Try again? Y/N ")
        if retry.upper() == "Y":
            guesses_left = 3
            random_number = random.randint(1, upper_limit)
            upper_limit = choose_difficulty()
            continue

        elif retry == "N":
            break