This appears to be one of the most imprecise errors a web server can through at you. More or less, it means “I tried but I can’t serve and I can’t help with more precise information”. People and forums tend to say that this may be related to database issues or .htaccess issues. But not much help to be found.
My recent experience has led me to find useful data, mostly under the form of checklists of things to verify on the web site when looking for the origin of such an error. So, I recommend you try relying on these resources:
- WordPress erreur 500 (by Kyodev, in French)
- How to clean a WordPress Hack (by Sucuri, also provider of security services that I did not evaluate myself)
- How ot fix the 500 internal server error on your WordPress website (by Lyn Wildwood,)
- Damm you WordPress 500 Internal Server Error