Maybe we can narrow it down - does it actually seem to function and stop on a known active frequency, like NOAA, even though there is no sound? If it doesn't stop, then one common cause is a broken bnc center conductor wire.
If it does actually stop, but with no sound, then have you tried listening with headphones? Since speaker audio is routed through headphone jacks, it is not uncommon for the contacts to oxidize over time without any exercise once in a blue moon.
What do you listen to? Conventional, Trunked, or a mix of both? Are you accidentally in the TRUNK mode when you are scanning conventional channels? If you listen mostly to trunked, is it possible that the system itself has changed? I had one of those a long time ago, and seem to remember that if you hit the TRUNK key accidentally, you can't scan convention channels.
It was also my first introduction to the fact that when trunking, it seems like the squelch is broken producing no sound when wide open, unlike conventional channels where the squelch is operating in a classic fashion.
