The water in the pump will heat up. The heat will convert to steam. Excessive heat will result in components melting down. First the Pump basket. Then either the pump pot (where the basket is) and or impeller. Next, the PVC plumbing to the inlet of the pump.
But that's all.
Oh, wait, the motor will shut down after a while (usually after the damage is done), due to a high temperature cut off in the motor, so you may think the motor is bad. Let it cool back down by leaving it off for a few hours.
But that's the worst that can happen.