You just upgraded to the latest version of WordPress for your blog, or you just updated a plugin, and all of a sudden your entire site is gone and you’re freaking out because all you get when you try to login, or go to the homepage, or access any page on your blog, is an error saying “500 Internal Server Error”.
Your problem may not be the same problem I had, but maybe what fixed my blog will also fix yours.
2. From the unzipped directory of the latest version of WordPress, copy all the files in the root directory (that is, all the files other than those in the wp-admin, wp-content, and wp-includes folders). If you haven’t already upgraded to this latest version, then copy all the files in the root directory, plus the wp-admin and wp-includes folders.
3. Paste those files into your problem blog directory, overwriting the files that already exist there.
4. Upload the new files. With any luck, everything will work perfectly again. At least that’s what worked for me. No more 500 internal server error.