KARL O
dont mean to be cheeky but have you checked the fuse if its not this then its prob a loose connection

Tedruski
If this is the drywall sander with vac system, it probably is the motor brushes. Some newer tools have auto shutdown if brushes get worn too much. If this is a palm sander or random orbital sander, they aren't made to sand drywall- the dust probably penetrated into the motor and brushes. That dust really is hard on tools, clogs air inlets and causes them to overheat. Blow out with low air from compressor blow gun, take to tool repair shop. Brushes and replace labor $20-$30.