For future reference, you should backup your BOINC folder once a week or so, to guard against a computer problem. In this case, you would also have needed to have at least one backup on cd/dvd, to save the key.

You can\'t, because you can\'t merge accounts. (Two different \'people\'.)
Go back to your old account, and use the option near the top of \"Your Account\" page to change your email address. The project web site doesn\'t know that your old email account is no longer valid, so you can still use it to log in and change it.