Site down - 404 error

My sub-domain site on this network is down (returns 404 error). Had been working fine for a couple of years! I've tried all the usual (disabling all plugins, deleting htaccess, re-setting permalinks etc.) but can't find the problem. Help!

The site is mapped to - the domain DNS includes the wildcard and the mapping has been working fine for years too. Nothing has been changed.

Support access has been enabled.