If you forget your email for MySpace, the best thing to do would be to visit the MySpace "How Do I Get Back My Account" page. This page has steps to follow to recover your MySpace account even if you don't have your email or password.
Fortunately, MySpace has a variety of resources to deal with lost or forgotten accounts. If you remember your email, account name or other sparse details, that should be enough to recover your account. The MySpace help page has forms that you can fill out to recover the remainder of your account information. Unfortunately, if you forget your email entirely, you may be required to fill out a "Declaration of Ownership" for your account. This, too, can be found on the MySpace account recovery page.
Filling out this form requires entering as much account information as you remember, as well as a current email address. If all goes well, MySpace can get back to you in 72 hours with your account information.
If you remember the email but can't access it anymore, you should check with the service that provided that email to see if they have procedures to follow to access your email again. If all else fails, it is also possible to contact MySpace directly and ask if they can provide you with more detailed assistance.