It seemed to have occurred randomly. I checked the services on ArcGIS Server Manager and nothing appears off. I have tried stopping and restarting services. AGOL still cannot connect to anything on our server and it's been almost 24 hours. Hosted feature services are the only layers still functioning.
In online map viewer:
Solved! Go to Solution.
Did the IT alter the domain setup of the OS Server or the ArcGIS Server? Could you be specific what was altered and on which server? I am facing exactly similar issue and trying to pinpoint the underlying problem. Your initial response is a good lead already. Thank you.