As 500 series errors come into the generic category, therefore diagnosing them is quite hard. But for an expert having some smart representations and deep know-how of its characteristics, tracking and finding the error is not a very hard nut to crack. Talking about WordPress Error 500, when you are about to diagnose the error, sometimes you get hints from access logs.
If you face the error, first you have to change the name of existing plugin folders so that they can be disabled. If you see a problem that does not appear again, try to change the name of plugins one by one as it was previously.
If you find there is a possibility to change the theme on a temporary basis, you can also try doing this. Ideas from your hosting provider while fighting with your website are also fruitful.