If you mower is still running well then the blades may be blunt and in need of sharpening or replacement.
Your first check should be fuel, if you haven't used the mower a lot then the fuel may have gone off. Petrol only has a certain shelf life, especially if you have put fuel with ethanol in it which tends to hasten the speed at which it goes off.
Having your lawn mower serviced involves a small engine mechanic checking that the engine is running in peak condition and making adjustments if required to ensure smooth running. It also means checking that any of the safety features are functioning and that the mower cuts grass well.