Go back to PhpMyAdmin, go to the 'SQL' tab, and paste this:
SELECT * FROM wp_users LEFT JOIN wp_usermeta ON wp_users.ID = wp_usermeta.user_id WHERE meta_value LIKE '%administrator%'
You should get a list of all the users with the 'administrator' role. One of those should be your account. You ought to recognize the email address or something. Then use the password reset function at /wp-login.php to straighten things out. Be sure to check that the email address in the database is an address you still use. If not, change it so that you get the reset email.
How was it that you could change your password using PhpMyAdmin if you don't know your username?
As far as what is happening? Its hard to say. WordPress actually uses a salted MD5 but my understanding is that it should still accept a straight MD5. Maybe there is a bug that prevents that. Or there is a bug in PhpMyAdmin's MD5 function. Or you made a mistake. Its hard to say.