An AirTag remains linked to the Apple Account it was originally paired with, even after it has been reset. To remove this association, you must first remove the AirTag from the original Apple Account before pairing it again. (How to reset your AirTag - Apple Support)
You are probably signed in to a different Apple Account on your iPhone than the one originally used to set up the AirTag. Otherwise, your AirTag would show up in the Find My Items list.
If the AirTag is in the Find My Items list but not reachable, please make sure that you have the AirTag nearby your iPhone and your Bluetooth enabled.
TL;DR To recover your AirTag, you should try to find the Apple Account that was tied to your AirTag.
Joe