Check the power cord or battery if the unit is not turning on. Inspect the battery for a good charge by plugging it into a charger or by plugging in the unit using the power cord. Check the power cord for signs of damage.
Blow dirt, dust, grass and other debris from the motor housing using compressed air if the unit is running hot or struggling to maintain power.
Remove the motor housing by taking out the screws along the edge. Remove dust and dirt from the cooling ports if the unit is running at low power.
Inspect the inside of the motor for corroded terminals if the unit will not turn on or shorts out. Replace wires and terminals as needed.
Apply the grease recommended by your owner's manual if the unit is running hard or making a grinding noise. If the grinding noise continues, take the unit to a repair service.