We hear this one quite a bit. There is usually no need to worry and a simple answer.
This is often the reason...
The belt may have come off or, possibly, broken, which will cause an overload. This can happen if you have gone too fast or forced the machine through grass which is too long or wet.
Customer Care Tip: On the first cut of each season, set your blades to the highest setting and only cut 1/2 a deck width to start. This will ease everything in and avoid those issues.