I ran in to this problem today, so I am posting it here with the hope that it saves you some time...
It seems I was a little hasty when publishing a new custom theme to my site. When I switched to it, all of my content was gone and I could not log in to the Admin screen to change it back. It quickly occurred to me that I had renamed a number
of Zones so now all of my content was in Zones that no longer existed.
In order to fix the problem I had to find the table in Orchard that saves the theme name which turns out to be: Orchard_Themes_ThemeSiteSettingsPartRecord I updated the string value to a known good theme and
after restarting the app I was back in business.
P.S. If you have the multi-tenancy module installed then the correct table will be prefixed with your instance name, e.g.: