Your switch may be bad or came disconnected. Or your PTO could have came unplugged, or it went bad.
Most common problem with older models is that the blade drivebelt is loose or has slipped off one of the drive pulleys/wheels.
Please provide the model number when asking a question like this. Different models use different techniques. Craftsman LT1200
check to be sure the blades have free movement. usually if you have this problem it is caused from something stopping the blades from rotating therefore it stops the engine
Husqvarna
Check your shear pins.
Two types are listed, on mulching(134149) and one hi-lift(138498.) These have the star hole in the center.
A person can get an owner's manual for a Craftsman lawn tractor by contacting the Craftsman company. The phone for customer service is 1-888-331-4569.
1997
MTD
1997
2006
There are two blades on that model. It has mixed reviews due to placement of some features but it is attractive to some people because of the large mower deck.
Try searspartsdirect.com