How can I get my original Blackberry Chat back?

not sure why that should happen but if you have backed up data on pc simply restore bbm data, if you havent backed it up its a good idea to download blackberry desktop and do so, that way when any thing like that happens you simply connect to pc and restore it all. and you dont have to restore all data you can just tick for instance bmm or contacts. you will always have same pin its unique to each phone, have you tried a hard re-boot (remove battery with phone on, wait 20secs and replace) this sometimes resets things.