If your sub pump has a float switch check that it is functioning correctly.
It's quite common for them to lose buoyancy over time which will stop the pump.
So it's worth checking if it is floating or laying on the bottom of the tank.
If it's laying on the bottom of the tank, lift it up while the power is On and see if it kicks into life.
If it does then you can remedy the problem by adding buoyancy to the switch by cable tying a foam poly ball to it.