On the side of the motor unit you will find the 'down limit' control. Turn it a couple turns in the opposite direction of the arrow printed on the cover and check. Some tweaking one way or another may be necessary. What I think is happening is the door is hitting the floor and the safety senses it has hit something and reverses it to the open position.

I have Craftsmen garage door opener (3yr old). There is
about half an inch gap between the door and the frame when
shut. Not all the way, just about 3feet from the floor.
The gap brings in snow, it gets windy and cold too.
How do I minimize the gap? Can I do adjustment myself
or can a professional garage door repairman can do it?
Please offer me suggestions.
andy2007

andy I think you need a door repairman out.