It likely needs to be re-programmed, or the batteries need to be replaced. If the batteries have been replaced and it's still not working, then try reprogramming it:
This depends on whether or not you have a universal remote control, or just a standard one that came with your television. For this, you will need to goto RCA's website:
http://www.rcaaudiovideo.com/
Type in your Model # into the search bar, located on the back of the unit. This will take you to a support page where you should be able to find some information on reprogramming the remote specific to your unit. If you can't find it, I would suggest calling RCA's technical support.