Enabling aggregate and compress CSS files resuls in vanishing theme I used this module for more compression on css and js files but it does not work for me! so I disable and remove it entirely.
After that when I enable aggregation of css and js files, my site's theme will disappear! even my administration theme! what should I do now?
Do following steps: 


Again place advagg module's folder in your module's directory. Usually (sites/all/modules)
Go to database and truncate all cache tables starting with "cache". Do with caution.
See if site works. 
Otherwise if you are having drush installed, check command "drush status" if it works. And clear the cache throudh drush again by command "drush cc all". 


Hope this will help you. If not reply in comment I'll assist you to make your site work again.