This has actually happened to me twice. After a lot of frustration and cussing I googled it. Hold the power button and the home button at the same time for about a minute and it should bring it back up.
This also happen 2 me yesterday, the Apple rep said I got water down in the bottom of phone when I put the phone n the cup holder...I asked him is it a IOS-5 problem...he said no, but it looks like it....