Our sandbox account seems to have disappeared. Accessing the regular address gives a 404 and there are no accounts in Account Central.
We were using it for development and maintenance of an integration.
I know you said you’ve been using it for maintenance and dev but have you recently actually logged in? I don’t know the time frame for it but they will de-activate after a certain number of months. Outside of that I would say contact support (phone is probably better than chat right now due to them staffing partnercon).