First, make sure the outlet pipe is not frozen shut or plugged and that it directs water away from the house. Unplug the sump pump. Remove the lid (if the sump has one) and use a flashlight to check if the sump is clean and that the pump intake is not plugged.
For pedestal pumps, the intake is on the top of the pedestal in the water. For submersible pumps, the intake screen is just below the motor. Normally, the intake is visible even in water but if not, use your fingers to make sure the intake is not plugged.
In some sumps, the tile inlet or inlets are near the bottom of the sump. In this case, temporarily plug the inlet or inlets. Next, make sure the pump is plugged in. Then slowly pour water into the sump. Try to simulate the speed that water normally would flow into the sump.
Watch the on/off float switch’s action and listen to the pump. Make the pump turn on and off at least twice. If the pump doesn’t sound right, fix it as soon as possible.