If the main light flashes 10 times when the door reverses it is a sensor problem. Either they are not aligned properly or wired wrong. There is also always the possibility they could be bad. If the door is reversing when it hits the floor then the down limit needs to be adjusted.

Thank you don for your suggestions. Unfortunately they didn't work. I called a Sears garage door technician. He found that the guys that had installed my door had put the chain on wrong, so the sensors couldn't figure out where to start and stop the door. After he made the adjustments to the chain, everything worked fine. Thanks again for your assistance.