I turned on Optimize CSS files in admin/settings/performance and now my site has no styles at all. The directory is writable and I see that Drupal created a few css files in there. /home/drupal/sites/mysite.com/files/css
thanks
This happened to me because of an .htaccess file planted by Drupal on the files directory. Also, all images on the files directory were gones as well.
The .htaccess file contained the following three lines:
SetHandler Drupal_Security_Do_Not_Remove_See_SA_2006_006
Options None
Options +FollowSymLinks
The file was not compatible with the security settings of the client's host. (Further information available on here).
Renaming the file solved the issue. Other measures like changing the tmp directoy or making sure there is enough disk space or installing the advagg module didn't help.