I have a similar problem. I have a WordPress blog installed at http://professorgrow.com. It is installed at the root, not in a separate folder. I created a mySQL database for it. I have been running it fine since February.
I decided to try a different theme. I downloaded a theme called "Structure" that is a child of a theme called "Hybrid." I followed their directions exactly -- downloaded and installed (but did not activate) Hybrid. Then downloaded and installed Structure. It said for me to activate Structure only, which I did.
Now I cannot see my site at all!
I cannot even get into admin to fix the problem.
I used FTP to delete Hybrid and Structure from my wp-content/themes folder, thinking it would cause my site to revert to default theme and let me into admin to activate the theme I had been using, but I still cannot see my site!
Any help will be greatly appreciated!