Thanks to this post, I have finally solved my WordPress login issue:
Last May, I noticed that I had only posted 4 things in the past six months and had not been doing regular upgrades and was basically completely out of touch with the web world and the art of regular writing. I got excitable, wrote some stuff, decided to upgrade WordPress to the latest version, couldn’t log back into the admin site and have been locked out ever since. I spent two or three evenings spread across the last 6 months trying to solve the problem and decided it must be a weird Mac thing. I had pretty much given up until I read this post and cleared the plug-in entry from the database.
The only remaining issue was to try to get my uploaded media content working again, but I solved that by linking to it elsewhere. So now I have a working blog and all I have to do is start writing!