Firstly, open your tag, re-insert the battery, and then close the tag, making sure the battery is facing the right way. Sometimes batteries become dislodged inside the tag if not firmly clipped in place. If you still do not get any noise from your tag, it is most likely the battery will need to be changed.
If you do hear one beep, this means that tag has already been registered. This will either be to the existing handset or an old handset (it can only be registered to one handset at a time).
If you hear 2 beeps, this means the tag is unregistered and you should register your tag again. In this case only, you'll need to delete the tag from the button it was registered to on the handset and then register it back to that button again.