You have a problem with your transmitter/receiver pair. My guess is that the hand held unit (the transmitter) has the problem.
If it's an older model they're a relatively simple oscillator & "loop stick" antenna.
First things first, did you drop it ?
If it's older, the loop stick has a ferrite in it tuning it to the right frequency. If it's a newer model, then the whole hand unit might just be "kapoot" and you would have to contact the manufacturer for a replacement (or go to where you originally purchased it).