I had to use an ftp program, such as FileZilla to access the files and turn the plugin off from there.
From the wp-content/plugins/ directory in FileZilla I manually turned off all plugins, which meant I was then able to log back into the admin panel, and then I turned each one back on one at a time until I figured out which was causing the problem.
This should work for you if it is also a plug-in causing the problem.