The problem is the higher resolution, the PC and monitor aren't on the same page so you get that jumbled garbage when trying to work on the computer.
As far as rebooting goes, you didn't do nothing wrong there, the way you fix this is to boot into safe mode and change the resolution.
BUT!! If you aren't getting past the POST test (The Power On Selft Test, that the computer runs everytime you turn it on) It could be a couple of things.
Just for the heck of it, make sure all of your cables aren't loose.
Do you hear any beeps when the computer is booting?
...try it again,
Let us know how it goes and we'll take it from there.
Also, this might belong in the OS/Software forum.