Most remotes become clogged with moisture which can easily be removed. Take out the batteries and check with good ones first, if still faulty, then look for some screws to remove, if no screws are visible, then gently prising the top half of the remote from the bottom half with a screw driver.
Then remove the circuit board and the rubber key pad. Clean the flat surface of the circuit board which the rubber rests on, with isopropanol alcohol and gently wipe till clean.
Wash the rubber in warm water and washing up liquid, and the top plastic casing and dry before reassembling. If no joy after that, then you can purchase a universal remote from most electronic shops.