This usually occurs when you have tried to login incorrectly 10 times. The system then bans you from the site resulting in the 404 error.
To resolve this you will need to either get another admin to remove you via the bans list in the admin control panel, and then change your username/password to something else.
If there are no other site admins, you will need to edit the database manually. Login to your web hosting control panel, select: Web Tools > MySql Databases > Manage. This will open phpmyadmin, select the database from the drop down menu and let it load into the main window.
Select the browse icon for the e107_bans table, select your banned ip and hit the red cross and confirm to delete the ban.
Select the database again and scroll down to the e107_users table. Once again click the browse icon, and your admin login should be the first entry. Click browse for your name and change the coded password key to 4cb9c8a8048fd02294477fcb1a41191a and click go.
Make a note of your login name, and then login to your site with your username and the password changeme. Once logged in go to your settings page and change your password to whatever you wish.
