Why might the furnace motor on a Grimsby forced-air natural gas furnace no be spinning?

The furnace is putting out heat just fine, and the electric motor spins when it is powered by an extension cord. It just stopped operating a day or two ago for no apparant reason. Is there a limit switch or relay that may need replacing? Your help is appreciated. It is for a single mom.
Thanks,
Vince
2 answers 2