Hi,
The "Output Compression Level" is not validated on input and can cause the system to crash with a blank screen if an incorrect value is entered. Then there is no way to load the shop or admin.
To resolve the issue you need to edit "/core/lib/response.php"
LINE
132 $this->addHeader('Content-Encoding: ' . $encoding);
133 $level = 9; <-- ADD THIS TEMPORARY LINE TO GET THE SYSTEM WORKING
134 return gzencode($data, (int)$level);
134 }
Once you get back to admin enter a correct value for "Output Compression Level" and remove the above added line.
Hope this helps.
Raj.