Hi, Darcy. The first step is to turn off the power via turning off the switch or pulling the plug. It is in the electrical code on many areas that sump pumps have to be on theirown circuit.
There are any different designs and kinds of sump pumps so one answer for what to do next is difficult. My pump has a tube to a pressure switch. Sometimes algae or moss clogs the tube and it will not turn on or off. I have to pull the pump and clear the tube every couple of years.
Other pumps work off a float switch. There is a good possibility that yours is hung up with algae, moss or other debris. Again, you do the same. Pull the pump, clean off the crud, make sure that the mechanism is free and the float is not full of water and reinstall the pump. That should take care of the problem.