as gelado says above,,could be a fuse or bulb,,process of elimimnation,,,could need a new timer,,,walmart/bandq or homebase timers tend to blow out so you may need a grasslin [german made=best,, imho] timer or something like that,,good luck,,peace and regards 944s2You may have blown the fuse or something internal. Check the fuse first (unplug it before you do). If not, get the ballast replaced under warranty, or see if they can fix it for you if you're out of warranty. Otherwise you'll need a new one...
I'd unplug it and leave it to chill first, and then try plugging it in again. It may also be the bulb? I have yet to blow a ballast/HID bulb (knock on wood).