I would try to turn the engine over by hand if possible just to see if something broke or if it's locked up. That may be all it takes to break it loose. I'm wondering, also, if the belt is too tight, maybe pinched, preventing the engine from turning.
First thing I would do is try to determine if it is and engine or a transmission problem. Looks like when you set the parking brake it takes the tension off the belt going to the transmission. So you would want to set the brake before trying to turn the engine by hand.
If your living right it might just be a branch or twig caught up underneath in the belts and pulleys.