There is an option in the database that shows the Zenphoto release #. From that you can look at the change logs and/or the various releases to see which one matches. Not a simple thing.
But it sounds like you are still running the old version, so just look at the bottom of one of the admin pages and the version is shown.
If you cannot figure out the old release you can just install the new one. It should happily migrate from that release so long as it is not too old. (I believe people have migrated from as far back as 1.2.5)
You will have to discuss with your host how to restore your database and album files. Just put them back as they were and install the new version of Zenphoto.