If you have a 1 series, spray penetrating lube on the linkage behind the parking brake lock mechanism. I use use WD40 with the flexible outlet. Aim this in the slot where the parking brake lock lever is and spray away.
Did you get underneath and check to make sure a stick/something hasn't gotten stuck in the linkages?
I had a branch jam up underneath and wedge all in stuff, just enough to hold the brakes in place the next time I had pressed them and used the parking brake. I couldn't get the parking brake to go off since I couldn't press the brakes at all to release the mechanism. Once I got under and pulled out the wood bits that were in there (it was small sized stuff that had broken off) it worked normally again.