What helps
If a conversion went wrong, three things make it much easier to fix: what you were converting from and to, which browser you're on, and what actually happened - an error on screen, a file that wouldn't open, a tab that just sat there doing nothing.
Don't send the file. I can't do much with it anyway, and your files staying on your machine is the entire point of this site. That doesn't stop being true because something broke. Just tell me the format.
Asking for a format
Worth doing. Most of the list is here because somebody wanted something that wasn't.
Some formats can't be done in a browser at all - usually the ones that need a patent-licensed decoder the browser doesn't ship with. So you might get an explanation back instead of a conversion.
Privacy
Emailing me is the only way this site ever learns anything about you, and only because you decided to. No forms, no account, no tracking - the privacy policy has the details.