For some unknown reason, one of my AirTags cannot be located

... so your friend had it, and then, lost it? If that is the case, then (as others have already mentioned) another iPhone, with Find My Network enabled, needs to be nearby in order for this AirTag's location to be transmitted to the Apple servers by that phone.

However, if the battery is depleted or the AirTag is located in such a manner that its broadcast cannot be sent out, it will not be discoverable by any iPhone.

See:

How to replace the battery in your AirTag - Apple Support

Note: As mentioned in the linked article, AVOID purchasing replacement batteries w/ the “child guard” Bitterant coating.

They will not work in an AirTag.

Re: “… one of my AirTags cannot be located …”

Just to be clear, Rudgar’s and my own replies assume that you DO have the AirTag nearby and that your device just isn’t “seeing” it.

If that’s not what you meant (i.e. if you really DON’T know where your AirTag is located) please post back.

if your friend does not have an iphone with bluetooth enabled then it will be dead in the water, it's only means of communication is bluetooth and if no ios device is within range then it will not be able to pass on a location

maybe it ran out of batteries?

In addition to her bluetooth, your friend needs to have her “FindMy Network” enabled.

This setting is addition to basic “FindMy”