Just in case anyone finds this problem, here is a useful tip!
Renaming the Theme Folder
Using FTP, navigate to your WordPress blog folder, and into the “wp-content/themes” directory. Here you will see a list of themes you have installed with each getting its own separate folder.
If you do not know what theme is the current one, you can reorder your folders by date/time (Last modified in the screenshot).
You can of course just delete the malfunctioning theme, but that seems a bit extreme. Instead we’re just going to rename it to something else as shown below:
Once we do this, WordPress is no longer able to find the folder for the current theme and so deactivates it. We still haven’t restored our front-end yet, but our WordPress dashboard is now accessible. We can navigate to the themes section and activate a new theme.