"Appointment cancelled with no warning"

They had booked an appointment with me. I had confirmed it again over the phone. But they never called on the day. No explanation. Nothing. You cannot contact them because their phones are not working, their document upload is not working either. The complaints procedure does not give any information about how to contact with a complaint. They give you the name of the person to whom to address the complaint but no contact details. What am I supposed to do? Write them a letter and send it by post? Turn up in person in the time of COVID? Very disorganised. Helpless, really.