Q: Car won't start after changing water pump

It sounds like something was left disconnected during the repair. Without more information like whether it turns over or not, or if any lights on the dash are on, etc., I will not be able to give you a precise answer. Do a visual inspection of the components you replaced and the surrounding area to ensure something like your crank sensor is not unplugged.

Also check the state of charge on your battery to eliminate this as a cause. It is also possible that when that battery was disconnected during repair, the security system has been enabled and my need to be reset or re-coded with your key. Refer to the factory service manual for details on how to ensure the security system is properly functioning and information on resetting or re-codeing your key to the vehicle. A certified technician, like one from YourMechanic, will be able to perform tests to diagnose the starting issue more accurately.