Jools
When you sign on to this website there's a check box that says "Sign me on automatically" (or something like that). If you do that, it writes a little file to YOUR computer, called a 'cookie' that has your logon and password info in it. Some browsers like MS Internet Explorer will also sometimes 'helpfully' offer to remember on your behalf things such as passwords that you might type in.
Personally I NEVER, EVER select these options and I always tell my mates not to either, for exactly the reasons that you're experiencing now. If you always type in your password, you're far less likely to forget it, or you'll make a note of it somewhere. If you lose that cookie or change your computer, and you've not remembered your password, you've had it.
No consolation to you now, I know - but basically, if you've lost that cookie there's nothing that Mike will be able to locate for you to get your password back.
The other problem in this case is that the MS Communities sites don't have their own logon id and password system themselves, but use an MS global logon and password system for all of them (.net password). These cannot be administered by Community site administrators, so login names, passwords etc cannot be changed. All that Community site administrators do is set permissions to say whether you're allowed into the community site once you have successfully signed on to .net.