I accidentally delete a file from the mount device that too as a root. after deleting I couldn't even find a file in trash.
Can somebody help me to restore the deleted file. I am using Debian 7
If it's ext3/4 then try extundelete: http://extundelete.sourceforge.net/ For other filesystems there are other, similar tools.
I assume that you don't have any backups, right?
AND STOP USING THIS DRIVE ASAP!