Screen relosution wont change black screen full window mode..

So ive been playin league normal on this pc and then i got a laptop and went playinf on this one so my sibling wanted to play league (yes i had to teach her) so suddenly the screen couldnt play in fullscreen anymore the screen wouldve turned black and had to shut down the pc to get out of it.. but now i went in the settings and saw that the resolution is 1600:2400 and my scfreen is 1600:900 and when i changed it and tried spectating i got this error from my monitor that the current screen resolution isnt supported and had to change it to 1600:900 and went back in the game config and saw that the screen resolution was 1600:2400 soo i really dont know how this is happening i tried like 10 times but it wont make me change it.. it only happens when i spectate or go in game before i can close and open it as many time and then it stayed on 1600:900 but when i go in game it change automatically back..

anyone thougths about this? thanks in advance!!