Alright, the same thing happened to me as the above people.
After the "Congratulations! All the files have been upgraded to the latest version. Please CLICK HERE TO COMPLETE THE FINAL STEP." came out, I was forwarded to a log-in screen where only the [USERNAME] was available. When I typed in my password it would just re-direct to itself over and over again.
I have tried upgrading 2.6.1 to 2.6.2 with WP Automatic Upgrade.
I backed up the SQL file and when I got into the same mess as some of the above users, I immediately tried to revert back to the old database. However, the problem still lingers!
I then manually tried to upload the WordPress upgrade files onto my server and tried overwrite the WP Automatic Upgrade files that uploaded previously but that still doesn't work!
The problem exists in all of my browsers (Firefox, Opera and Internet Explorer) even with the cookies reset each time I tried to change the password.
I went into phpMYadmin, changed my admin password there and then selected MD5 in the drop down menu. It didn't work. I put in the new password and I just get the WP log in screen again and again.
I have tried the last resort Emergency Password Changer created by village idiot. However, the password that I created using this script did not work either.
What the hell is wrong with my installation? Can someone please shed some light on my errors? Thank you!