Damaged lid O ring Water level is below skimmer Strainer or skimmer basket is clogged. Closed valve in piping system. Air leak in the suction line. Filter is dirty Restriction in the return line. Strainer basket or skimmer basket is clogged.
Circuit breaker is tripped.
Pump is in off mode of timer controlled circuit. Motor terminal connections are incorrect. Motor shaft is locked by a bad bearing. Impeller is locked by debris. Wiring to pump is undersized.