take batteries out and reverse them. make sure theres a connection at each end, if not then use a staple, paperclip etc to make a positive connection. now press all the buttons on the remote. put batteries back the right way and test it. if it still doesnt work then go to remote control shop.co.uk. if it is a stuck button then point the remote into a digital camera, phone camera and see if theres a flashing white light. if not then theres no stuck button
Some remotes won't tolerate a battery reversal. It won't do them any good. In my experience, you can achieve the same effect simply by removing the batteries and holding a number button for 30 seconds. This discharges the internal capacitor and effectively clears any data in volatile memory.
But your symptom often occurs if liquid has been spilt into the remote.