I would first check to be sure that the slide or gate type valves have not closed by accident. Not all spas and it can vary by year of spa model if they are present. They would be little handles on each side of the pump area that should be all the way up. If the locking mechanism on them was not set or broke it would be easy for one to slide down while work was being done in control box area. A pressure switch is an easy part to replace and one of the parts most often replaced on a spa. Also you want to check the area of the filter cartridge where it goes into tub. Many times there is a screen type fitting (may be up in the filter cartridge) that needs to be cleaned of hair etc. The way you describe the sound it sounds like the water has stopped suddenly which would lead me to believe a valve has closed. With a pressure switch you shouldn't get a bang type sound.

Other places to look for obstruction would be the pump impeller. This week we pulled out a speedo from an impeller. We have NO idea how it got there and past the filter.

Slide valve in closed position

Did you just replace the control pack or just a topside control?

OK, then we don't have to worry about wrong connections.

Usually when motors sit they will just freeze up but that is more of a hum sound. Since it's giving the FLO error I would replace the pressure switch as it is not an expensive part.

However, while in there...if you can go to the back of the motor (WITH POWER OFF) and "turn" the motor shaft to be sure it is freely turning also that would eliminate the question of the motor hanging.