Wordpress plug-ins can be a common point of failure - particularly if you upgrade plug-ins and the new release has compatibility issues.
One of the common troubleshooting tips for Wordpress is to deactivate all plug-ins and activate them one by one. Which is fine if you can access the site. Fortunately there is a method to disable your plug-ins using FTP.
As long as you have File Transfer Protocol access, use your FTP client, or your host’s file manager to get on to the site. Navigate to the /wp-content/ folder where you will see a folder called plugins. Rename this plugins folder - I use "plugins.deactivate". Do this, and all of your plug-ins will be deactivated, since if Wordpress can't see them, it can't load them.
If one or more plug-ins caused the server error, the you should be able login to your /wp-admin/ once again and from there, sift through until you find the culprit.
To do that, login, go back to your /wp-content/ directory and rename "plugins.deactivate" back to plugins.
Now de-activate them properly using the wp-admin panel and scientifically activate one plug-in at a time until your site fails again. By a process of elimination you can deduce which plug-in caused the issue. RC