Systems info... post it. Can't help you without it.
I'm not even going to post the link. just read the stuff marked "must read" in the tech help forum... they're stickied for heaven's-sake.
1. make a backup copy of your Profiles directory so that you don't lose you hotbar setup : [MxO install directory]/profiles
2. reinstall your video drivers
3. if that didn't work, reinstall MxO
4. if that worked, restore your profiles directory