Why did my 76 chevy just stop moving?

i just bought this truck yesturday and i was driving my son to school and i heard a huge bang and the truck will not move now. the engine starts and i can feel the transmission shifting into all the gears. when i put it in park the truck still like its in neutral. it also makes a grinding noise when i put it in park while the truck is still running. last thing when i shift the truck before this happened i had to have the brake pushed in but now i dont the gear shifter will go through all the gears with no brake even out of park. if anyone could help me with this i would greatly appriciate it thanks for your time