If you can still access your account, go to your profile page, and then click the 'my logins' link. Associate any other working OpenID with your account, then remove the one that you can't access any longer.
Note: If you're logged in using Stack Exchange OpenID, you won't be able to set up a new Stack Exchange OpenID unless you log out first. If this is the case, use another service as an intermediate step, such as Google, Yahoo, Facebook or others. Once you've added the intermediate account, log out of your SE OpenID, then you can make a new one using whatever email address you like.
Now, if you can't log in at all, and no longer hace access to the email address needed to get a password reset link, go ahead and create a new, temporary account using working credentials.
Then, contact us (use the 'contact' link at the footer of the applicable site) and let us know. Make sure you include some, or all of the following information if you've given it in your profile:
- Your real first and last name
- The month and year of your birth
- The email address currently associated with your account
- The IP address used when you last accessed the site (or, an IP that you know you've frequently used)
- Any other details about your account that aren't public
- A link to the new profile you just created
What you're basically doing is telling us things about your account that only you would know, which helps to establish that you own it. Have you been contacted by a moderator for some reason? That's a great thing to include.
Once we verify that you own both accounts, we'll merge the new one into the old one, which effectively just copies over the working credentials so they get you into your usual account instead - the new one is deleted during this process.
This is a relatively fast and painless process, provided that you give us as much information as you can initially, which averts us having to ask for it instead of just acting on it.