I have tried contacting the sole (as far as I know) admin by Private Message and by email, and have received no reply to either after several days. I have even resorted to 'reporting' a post to see if that process is at least operating, and the answer is no.
I have previously encouraged the forum's owner to appoint additional administrators to protect against exactly this occurrence, especially as somebody (not me!) volunteered to take on the co-admin rôle, but unfortunately with no success.
Obviously it is not safe to leave the forum rudderless, especially with the requirements of the Online Safety Act needing to be met, so unless the admin resurfaces I will be forced to close the forum by the only brute-force method at my disposal (deleting files from the server).
This is the second successive occasion when the administrator has 'disappeared' without explanation. The last time it happened the previous owner had retained a 'back door' into the administrative interface and was able to appoint a new admin. Now that mechanism has gone.