You can also try taking off the battery for 30 minutes, then put it back in and turn on the phone. Some users say this method worked for them. Hopefully it works for you as well. Good luck!

Is it the "Emergency download mode"? It seems like a common issue with the Samsung Instinct M800. A lot of other users have also experienced this and after informing their carrier about the problem, their phones got replaced. Phone replacement may only be applicable if your phone is still under warranty. But if it's not, I'd still suggest you give your carrier a call so you can get assistance to have your problem fixed.