I got an error "The requested theme does not exist"! Oh! I checked my website and everything was fine. I have taken a long breath!
Now, what went wrong? I was editing the theme. I want to change something in style.css of the theme. I have tried to remove the theme info from style.css. Designer do not want to remove the theme information from there. I have restored it using ftp.
As I was surprised what has happened, I searched in on Google immediately and have found help here. You can visit there for more detail.
You can also restore stylesheet from you backup or download that same theme and upload the stylesheet from that.