Well... first thing is unplug it and short the terminals and see if the LED on the controller lites up. If so.. then the problem is in the tank. Make sure your connections on top of the tank have no corrossion...and make sure the float isnt sticking on the shaft.. if all else fails, you'll need a new low level sending unit for your tank. Contact Steve at SMC for help then.
Typically the switch on the float shouldnt go bad, but anything is possible. I would bet more a connection problem on top of the tank than anything else..
HTH