res

you changed your desktop resolution lower than 1440 and you monitor doesn't auto resize

you got a new graphics card and you extended your resolution beyond 1440 if your monitor can support more than 1440

those are 2 reason why it might of happened

might be a bug of some sort

you can try putting the games options into windowed mode at 1440 and see if it will go into 1440 windowed mode if so then try putting it back into full screen and see if it works, if not then you might have to reinstall the game