See Is your BlackBerry Nuked? - BlackBerryFAQ
. But beforehand, see if you can boot up in safe mode. If so, do a full backup. Then go back to the FAQ article.
To boot in safe mode, you will need to be running OS ver 4.6 or newer. Pull the battery for 30 seconds, replace battery, and then hold the back/escape key as blackberry begins to boot. You will see a notification that it has booted in safe mode.