Help plzzz! Problem wit screen resolution

Okk I don't kno all the specs 4 the computer but it's pretty old. And it's
windows xp.

My grandma had the resolution at 800x600 and it was realllly big so I wanted to get a better resolution so I didn't have to scroll up and down while playing games. I changed the resolution then left it a bit while the screen restarted. I came back to fine the screen was on sleep. I moved the mouse but the screen didn't come on. I then went and turned the moniter off then on. A message appeared in a blue box that said "pc setting correct?" it then went back into sleep mode. I've tried rebooting it to no avail. It boots then when it gets to the part where you Are gonna log on the same message apears and then the screen goes to sleep.

Plzplzplzzplz can I get some help??