Button doesn't work after build

I almost finished my first game but something weird happens... I have a Main Menu at the start of the game that works perfectly fine in Unity. The menu is like that: when the game starts you can choose "Play" (the game starts) or "Exit", and then an Image with text shows and says "Are you sure you want to quit?" And you can click "Yes"(the game quits) or "No"(back to Main Menu). In Unity, when I click "No" everything works fine (back to Main Menu and Image with text disappears) but after I build the game and I click "No" the image with text doesn't disappear...

(I tried to build the game in different locations but that didn't help)