If it was with Hotmail or Gmail or equivalent, it gets a little trickier.
With companies like this, it's difficult to get through to a "real person" to help you, so you'd have to do the hard work yourself. And there's not usually an option for "How do I find out my email address?" it is more set up for retrieving your password if you forget it.
- Have you tried asking friends/family that you may have emailed from it? They may have old correspondence from you which will have your email address on it.
If none of that helps, I would probably advise setting up a new email. It's really easy to do, only takes about 5 minutes, and if you haven't really been using your old one too much it won't be too much of an upheaval. I would recommend Gmail. It's super easy to use and the interface is very user friendly.