My AirTag was working and all of a sudden it does not display in the Find My app. I quit the app and re-opened it, though it still does not show. Is there some setting I can try?
You mean that it's missing entirely in "Items" or just the location isn't showing?
If the latter, maybe the battery is dead? You have bluetooth turned on, on your iPhone?
If the former, maybe your pairing with your Apple ID has somehow been lost. I'd reset the AirTag (pull the battery, then put it back in), then try pairing it again.