Can I revert to older version?

I have no idea how it happened, but my OC site was updated from (if I recall) version 2.3 to the latest 3.0.3.1 I did not do the update and my host Godaddy said it did not do it, so my best guess is a hacker but for what reason I do not know. Is it possible to revert to the older version? Since the update the site does not show up at this time, the error message is: Error: A theme has not been assigned to this store! I can however access the admin panel. I would like to restore it if possible as it obviously was not done completely or correctly (don't know which). If this is not possible, how can I fix it?

Restore from Server backups or ask hosting support to restore.

Contacted host (GoDaddy) to seek help and find out that their new installation set up has automatic updates. Urrgh! At least I wasn't hacked, I was a victim of the automatic update feature that I did not know about or authorize. Thanks for the help.