Murray 46" mower parking brake suddenly won't release. Why?

I have 5 yr old Murray 46" tractor mower. Suddenly, yesterday, the parking brake would not release--so tractor will not move, just sits there and burns the drive belt. Cannot seem to find the problem (very poor accessibility) and am about to disconnect the disk brake on the transmission so I can mow. However, when tranny is in neutral, the mower can be pushed.
This a first--the mower is a cheap piece of crap, but I have kept it running for five years--lots of problems, but not THIS one before.