Okay, I changed the theme and you are right: my site came back!
Now, how can I get back to my original theme, or can't I?
Great! Now, we need to isolate the error in your Theme.
The first step is to enable WP_DEBUG.
Using FTP, find the wp-config.php file in the root directory of your WordPress install. Open it using a text editor, and find (or add) this line:
define( 'WP_DEBUG', true );
(If it exists, it will say false; just change false to true.)
Then, activate your Theme, and report any errors that are displayed. (You may need to look in your browser source to find the error message(s).)