Processes in Microsoft 365 for setting up Office apps, redeeming product keys, and activating licenses.
If you are the buyer/owner of O365 you re-install via the MS Account associayed with the purchase https://office.com/myaccount
A quick repair is
Restart the PC, once fully loaded, repair Office,> Apps & Features>Select Office your version>Modify /Repair, once the repair completes re-test (Quick repair)
O365 when installed under a Win user admin account, and run from that admin account, auto logs in when the PC is started. There is no separate login for any component.