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I'm guessing you tried yoursite.com/user.php and yoursite.com/admin.php with no luck...
if you have direct access to the database (phpmyadmin), you can change ir back the in the config table of the dB.
look for _MD_AM_DTHEME and change the conf_value to default.
As a suggestion, from now on leave the admin section open in it's own browser window, change the theme, open a new browser and navigat to your homepage.... that way if your theme is hosed, you can just change it back in the still open admin browser.
