Google on "remotes" or "remote codes" and you'll find many companies who repair, or offer replacements, or universals. One of the sites I found that way some time ago suggested that often a remote can get stuck, and to "unstick" it you remove the battery and depress and release each key one at a time until each has been depressed. You do this in each mode - so for example if your remote has a switch of "ffn" key to enable keys to perform multiple functions, you need to go through each mode. I suspect this has the affect similar to shorting the battery terminals suggested above.
If this fails you can try one of the repair or replacement services, or use a universal remote - though you'll need to locate the code set used for your pre.
As for whether it's the remote or the sensor in the pre that's gone - the easiest way to determine that is to find a universal remote and codes and test it.
