This happened to me, likely because the game is 32 bit or something I don't know.
Solution: Start up the launcher > go to option > change the screen resolution to what suits your computer.
You may have to go into properties if it persists. Tell me and ill tell you what I changed mine to. :3 good luck~
Edit: I'm not on my computer ATM but when I am I'll tell you the actual settings if it isn't fixed by then. I also have it in window mode.