EEPP!!
I did a search on the web for anything that related to the error message I was getting:-
"Error 500 - Internal server error" and added +Wordpress
There was loads of people talking about a corrupt .htaccess file, so I carefully followed their instructions but nothing worked.
Finally I was able to log in to my site via FTP and renamed the PLUGINS folder as I'm addicted to plugins and I was sure it was something to do with that.
BINGO!
Suddenly I was able to access wordpress admin again. I then carefully went through all the settings and made sure my basic site was working properly. I needed to reset some of the reading and writing settings but after that everything seemed to work.
Finally I carefully moved over just the few plugins that I really needed and left the rest of the junk in the only plugins folder.
PHEW!
Panic over.