After replacing water pump temperature gauge is not working

A real good suspect is you unplugged the coolant temperature sensor and forgot to reconnect it. On older vehicles there were two of them; one for the dash gauge and a different one for the engine computer. Unplugging the one for the engine computer will be detected, a diagnostic fault code will be set, and the check engine light will turn on. That sensor has two wires. The sensor for the dash gauge has just one wire. If that is left unplugged, the gauge will stay on "cold".

By 2007, almost all engines use just one coolant temperature sensor. It is the two-wire type for the engine computer. That computer talks with the instrument cluster, which is now another computer. The dash gauge gets its information from the engine computer. Now both functions are done with that one sensor, so leaving it unplugged will cause both symptoms.