You need to logically separate the parts.
1. There is apparently electricity, but the pump isn't running. Figure out if it is 115 Volts or 230 Volts before proceeding to Step 3.
2. Make sure that red valve is open.
3. If the power connection is as your picture (float switch and power cord coming from tank), then take an extension cord and plug the pump directly into it (assuming that it is 115 Volts). NOTE: The pump cord is the one that is riding piggy back; the one plugged into the outlet is the float switch. If it now works, then the float switch is not working.
If there is only one cord coming from the tank, then you have a switch that is part of the pump and the switch may have failed. Go to step 5.
If there is no sound from the pump with step 2, then the pump is probably not working. It could be failed or it could be jammed.
If there is sound from the pump, but no lowering of the water, then the pump might be failed or the pipe could be plugged.
4. If it is a safe place to test the pump without making a mess, close the red valve and open that that union to disconnect the pipe. Have a garbage can to catch the drip. Then turn the pump on for just a second and see if anything comes out into the garbage can. If it does, then the pump could be OK and you could have a plug downstream.
5. Take the cover off (you may have to disconnect some pipes) and see what you have inside the tank. You probably want to pull the pump at this point and take it outside to inspect it. See if there is anything in the inlet, such as a T-shirt or a sock.
6. Test the pump in a tub of water to see if it works. If you can't make it work in a tub of water, when plugging it into an extension cord, then it is probably failed and at least needs some maintainence.
At this point you should have some additional information to help you decide what you need to do.