Basicly your rear caliper locked up with the e-brake mechignizm.
Most likly the right rear .Take a hammer and look for were the e-brake cable links to the caliper .Try to bang it back into the open position.
You will need to replace the caliper. There is no way to fix it. Until then you can just leave the car in gear for a parking brake as long as you don't pull the e-brake it wont lock up again.
What happened was rust got into the system and it gathers up at the right rear caliper until the e-brake ceases to work. When you get the new part flush the entire system after you replace the caliper. One time I didn't do this and the same thing happened in a few months.
I have had this happen to 3( 2 MK2's , 1 B3 passat )of my dubs. Rule of thumb I learned is to change my brake fluid no matter what evey 2 years
