500 Internal Service Error, tried plugins rename, what next?

I recently installed the Auto Post Thumbnails plugin on a site and got the 500 Internal Service Error message, keeping me from being able to access either the frontend or wp-admin for the site. Auto Post Thumbnail was the only plugin recently installed, and I’m sure it was this plugin that was the issue, because I had trouble with it before and turned it off. My hope was that upgrading to the newest version of WP would do the trick, but that was a big mistake.

Before the 500 Internal Service Error, I began the process of having the plugin go through and pick Featured Images. It got stuck at 66%, so I left the house with that running, hoping with patience it would be done by the time I got home. Next morning, 500 Internal Service error and no access to WP front or back end.

Having read through other posts on this topic, I’ve gone into FTP and deleted the plugin as well as renamed the plugin folder to plugin.hold, and just for good measure, renamed the themes folder to themes.hold (though I’m sure it’s this plugin to blame) so it looks to me like something has gone amiss with the database.

What are next steps to take to resolve this issue?

Please help!