there are lots of online store for replacements. Here is one I found that may be of assistance.
http://www.gatehousesupplies.com/rem...FUQb6wodhA0yDw
As Dan says, might just need to be reprogrammed. I had this happen a while back and was a quick fit by hitting the program button on the motor and then the remote.
sears also sells them and may be worth looking into.
hope that helps !
jeff