http://forum.xda-developers.com/showthread.php?t=2602956
If you are using the Stock Rom one of these should help.
Tap, Hold for 10 seconds
Tap, Tap, Hold for 10 seconds
Tap, Tap,Tap, Hold for 10 seconds
etc...
Some say to plug it into a usb port on a computer for 10 minutes and then plug it in...