If your garage door opener is not working you may need to replace the batteries, you should be able to access it on the back and get the old battery out and replace with the new. If that does not work I would contact the company who made it.
First off, figure out if you can exchange or return the item to the store. If not, see if you have a warranty with the manufacturer on this product. If so, call the warranty number and they will instruct you what to do next. If not, call a technician.