Um... can someone help me out on this?

I'm an idiot when it comes to computers, and i don't really know what to do in a situation like this:

My screen is 1600x900, but i accidentally set my settings inside a game of league (custom) to 1600x1200, so it's quite a lot out of range of my screen.

Like 75.1/60 Khz.

what could i do to change it back?

EDIT: nevermind guys, it fixed itself after a bit of pulling my hair out and restarting my pc.