There isn't a blockage or water in the line? Have you tried to take the air lines off and run it without any pressure on it? Sounds like the motor may be done. But if there is to much back pressure it might run a second then start humming. Do you have a check valve on it? Maybe water siphoned into the line after you cut it off and the motor doesn't have enough torque to clear the lines.

Just some thoughts. Pull the lines off the pump and try it again. If that works, pull the diffusers up, turn it on then let them sink again.