What to do when says Tuner not available?

...Been using for a few days on Firestick 4k. Yesterday got Tuner Not Available on OTA channels and subscriber channels. I rebooted a bunch of stuff and got it working again but I'm not sure what I did that actually worked. Why does this happen and what should I do if it happens again? Thanks.

I have seen this happen when the disk for the DVR had gone to sleep so you could check that.

This happens to me from time-to-time on my Fire Stick 4k. Sometimes, if you just wait 10-15 seconds it resolves itself and the GUI comes up. If not, then force-stopping the app and re-launching it usually fixes it.