Friend's account suddenly has Empty Watchlist

I had a power outage and when everything was restored and rebooted I have a friend account saying their Watchlist is empty. If they go to items that they remember being in the Watchlist it shows already added, but nothing is showing up.

What could have happened or what could be wrong with their Watchlist?

I don’t use the Watchlist so have no other accounts to compare with.

This has probably nothing to do with your server. Watchlists are stored directly at plex.tv
An empty watchlist is most likely caused by the user triggering a complete deletion of their data.
OR
by using a completely different user account.

As Otto said, this has nothing to do with your server rebooting. If your server was offline, they could still log into their client and view their watchlist. It wouldn’t link to your offline server, but they ought to still be able to view it.

Watchlists have been finicky for me lately. On my Nvidia Shield, it can show up empty, but it’s either because the app is acting up, or Plex itself hiccups for me right when I try to access.

Ask the user to swap to another Library view, then swap back to Watchlist. If that doesn’t cause the watchlist to populate, then reboot the player. If the issue still persists, let us know which client they are using.

How would one go about triggering a complete deletion?

They are using the Android TV app and they did reboot the TV, but that didn’t fix their Watchlist.