Hi @Eamonn Doyle
If you still have access to the Microsoft 365 admin center, you can recover the affected user's account by following steps:
1.Confirm the user account is active and sign-in is not blocked
- Sign in to the Microsoft 365 admin center.Go to Users > Active users.
- Select the affected user.
- Check that the account is still active and check whether Block sign-in is enabled. If Block sign-in is enabled, disable it and save the change.
2.Reset the user's password
From the affected user's account page, select Reset password and generate a new temporary password.
Provide the temporary password to the user through a secure method. The user should then sign in with the temporary password and create a new password when prompted.

3.Revoke the user's sessions
After resetting the password, revoke the user's existing sign-in sessions so that previously issued authentication tokens are invalidated.
Please refer to this guide Manage authentication methods for Microsoft Entra multifactor authentication - Microsoft Entra ID |…
As this is a Microsoft 365 Nonprofit tenant, I recommend contacting Microsoft Support directly through the Microsoft 365 Admin Center or the Microsoft for Nonprofits Support Portal. You can create a support request and select either email or phone callback as your preferred contact method. A Microsoft support engineer will follow up with you and assist further.
References:
I hope this helps.
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