I had just upgraded to v21 from the last v20.x. I ran it and played some live music streams. But then just a bit later I tried to play a TV series, then I got the prompt that the media wasn’t available anymore and it asked if I wanted to remove it. Of course I said no.
I exited Kodi, went to This PC and opened my mapped network drive, it took a bit to power up my NAS drives, then everything showed. I ran Kodi again and was able to select the show I tried earlier.
I’ve never had Kodi complain that my media was unavailable because the NAS drive wasn’t powered on. Usually it pauses while the drive powers on, then it’s accessible. So I’m not sure if this is a v21 bug or not, so I’m just making note of it here just in case others have the issue, then I suppose it could be considered a bug.