When i replaced my heater core on a 1995 Ford Escort my car won't start now?

check the timing belt if it has broken or stripped its teeth this would cause a complete breakdown u can check this by removing the cover on the end of the engine.there are electrical issues therse would need checking 2 generally its to do with the earths the earth wires in and around the engine bay if and when they give trouble they can cause a non start.if not you will need to get your car to a garge so they can put it on a diagnostic machine .