This message is for newbies who like me may be stumbling along with huge gaps in their knowledge - learning by doing but not necessarily understanding what they are doing ......

I had this problem and still don't know what caused it, but after changing my username and password in my database (via cpanel & phpmyadmin > ..._users), I still couldn't login. I tore my hair out in frustration, combing this thread and the web for suggestions, trying everything I could find, then discovered that if you change your username in the database, DON't use MD5 to encrypt it - only do this for the password.
This worked for me. I hope it helps someone else.