My iMac boots to a Guest Login

If your Mac was reported lost using Find My, it will only allow guest access. As soon as a person logs in to the Guest Account and activates WiFi, then the location is reported to WiFi. This is the way it protects your personal data while still being able to report its location.

Have you previously reported it as lost?

Did you buy this Mac new from a reputable retailer?

If you purchased it used and this is how it behaves, that is an indication of a Mac that was not prepared properly for transfer of ownership and you may be facing a Kobayashi Maru scenario.

The Mac should have been prepared per the guidance in this support document:

What to do before you sell, give away, or trade in your Mac - Apple Support

FYI, You've neglected to inform us of some very important details. Many times contributors will skip posts lacking details because at the time we may not actually have the time to play 20 questions. I saw your post the other day and did not have time to ask questions, plus you never answered @Mac Jim ID's question. Not answering a contributor's question can also have us skip the thread. Keep in mind we are just other regular users volunteering out time to assist people here. Apple is not here on these forums.

Did you just acquire this Mac?

Have you been using this Mac normally? If so, what has recently changed or what led up to this moment just before the issue occurred?

What is the exact model of your Mac and version of macOS being used? You can get the exact model by entering the system serial number here:

Check Your Service and Support Coverage - Apple Support

Is Filevault enabled?

Do you have any other macOS user accounts besides your account which is currently missing?

+1 👍🏽