One of my members has inexplicably and without notice, deleted his own account, or so it would seem. Is that possible? and can it be reversed?

His posts now appear as guest and all his posted photos have disappeared too.

Hello yes a member can delete there own accounts this can be done in two ways if you have it set to delete the accounts on your ACP it will have the settings on the members profile to do it or there is a code that is gave to bypass your settings but we don't really like to give this out but lots already know the code and use it.

By law everyone has to he right to delete there own account and there is nothing you can do about it.

AS for you getting the account back well that can also be done all you have to do is do a backup of the members just before the account was removed for just the accounts then it will come back but if you do that the member may get mad at you as they want ed to leave and they have all the rights to close the account.

Wha I did on my site is to ask the member if I could change the name and email account on the profile that was the posts was still there but you could just delete the account and all of the posts linked to that account.

Personally I would follow the advice Ape gave you by making changes in the AP so members have to request the Admin to delete their account.
Save yourself a lot of frustration and leave the deleted member as guest. If the member was a regular forum participant, members will still know who it was.

Thanks guys, I have it sorted now.