Q: I replaced the water pump but all the coolant leaked out - 1996 Chevrolet Cavalier

If there was no leak prior to the water pump replacement, it is possible that the gasket sealing area between the water pump and the engine block has a leak. This is easy to find if you refill the system and use a pressure tester. If the leak is at the gasket, the pump will have to be removed. The no-start situation is odd as, even if there is no coolant in the engine, although you should never attempt to start the engine with no coolant, the absence of coolant would not prevent the engine from starting and running.

It is possible that in the course of replacing the water pump, a part of the engine wiring harness that is attached to a key sensor was disconnected and that is why the car won’t start. If so that should be easily resolved. I recommend having a certified mechanic, such as one from YourMechanic, come to your location to find the leak and repair it.