2HP AO Smith on an FP5182-08 Flotec pump did not resume irrigation during manual switch over to another spray field. While checking it I found that the hum (attempt to start) apparently shut down by the thermal switch. While separating the motor from the pump I fount the reverse threaded slotted screw completely out of the center of the impeller. I could not detect any sign that the screw was jamming anything. The motor would not sustain impeller spin when I hand started it. I do not detect bearing drag.

SOURCE: finding Flotec impeller for a flotec pump modle FP4012

WW Graingers. Call them with model and serial #s. If you do not have an account with WW, These guys have what ever you need:
http://www.pumpagents.com/partpages/FlotecPumps/FP4012.html

SOURCE: My new AO Smith 2HP pool pump won't pump water, just hums

I'm guessing that you replaced the motor that goes to the pump, not the pump itself. You know the motor works just fine. Is the motor rotating in the correct manner(clockwise or counterclockwise) in relation to the rotation of the pump? Have you checked the pump to see if the impeller is working/rotating? Is the motor hooked up to the pump correctly, so that the motor is turning the impeller? Are all connections to the pump tight, so there are no suction leaks? Have you primed the pump and purged it of any air locks?

SOURCE: REMOVE IMPELLER FROM SHAFT A O SMITH PUMP MOTOR HIFLO

The impeller is threaded onto the motor shaft, take the back cover off and use a wrench to hold the motor shaft, then turn the impeller counter clockwise to remove.

SOURCE: My Flotec Model FPSC3200H-01 1/2hp

Well I know it a dirty job but someone has to did. Unhook the pump from the plumbing and take the pump out so you can see what fell into the sump pit and is lodged in the impellers.
If you have it out already turn the impiller backwords and see if that frees it up.