How long have you waited?
Did you try another, longer battery pull?
If it keeps happening and you have waited long enough to be sure it isn't going to boot...
Remove the microSD media card, if you are using one, and try again. If that doesn't work, see if you can boot in safe mode, and then delete that app which was hanging up on you.
See KB17877-How to start a BlackBerry smartphone in safe mode
for instructions on safe mode. You'll need to have OS 4.6 or newer.
If you can't boot in safe mode, you'll need to wipe the device with JL_CMDR and resintall the OS. See the FAQ for the two articles on using JL_CMDER. Posted via BlackBerryForums.com Mobile