Push the pedal in a little bit, then pull the release handle, then try to manually pull the pedal up if return spring doesn't release it.
Failing that, just go under the car and cut the ugliest/rustiest/most frayed cable you can find under there in half, which will release the tension...it was ugly, so it needed replacement anyway.
Then with tension released you can more easily figure out exactly what's stuck. Pedal? front cable? rear cables? mechanism inside rear axle drums?
