Some ideas:
Can you still get in using an AVD client? Do you have someone with owner access to the VMs? If yes to both, you can assign the Virtual Machine Admin Login role to a person's Entra identity for the VM's or Resource Group that contains the VM's. Next have that person login using AVD and they should be able to perform admin tasks within that session, to include opening a cmd prompt window as administrator. From there you can run lusrmgr and set the local administrator's password or add a new local admin user.
If you have privileged access to Microsoft Entra ID (at least Cloud Device Administrator), you can try using LAPS and get the password there.
- In the Azure Portal, go to Microsoft Entra ID
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- Then go to Devices
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- Under My Feed, click on See all devices
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- Find the Device by name that is your AVD session host, click on the name
- Then click on Local administrator password recovery
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- lf you don't see a password there, you will need to enable LAPS - https://learn.microsoft.com/en-us/entra/identity/devices/howto-manage-local-admin-passwords