Over the winter, rust may have accumulated, which prevents the pump from starting up or makes grinding noises when it is switched on. If the rust deposit is not too bad, this problem can be easily fixed.
The cover for ventilation in most pumps has a small recess behind which there is a screw. Slightly loosen the screw with a screwdriver and spray with lubricant (such as WD40). If the rust has not dissipated too much, the pump will work again.