Electric sump pump sometimes fails to engage / kick on
The float should travel freely up and down on a vertical rod.  

As the float rises toward the top of its travel, it operates a switch.  This is the switch that turns on the pump.  

Raise the float manually.  The switch should operate and the pump should turn on.  If it does not, it is the power supply, the switch or the pump.  

It should be obvious just by looking at the float whether it is floating or not.