The error means the server crashed. Usually that’s because of a syntax error in a PHP file.
If you check your PHP error log, it will show you exactly where the error occurred. Or, to rule out another plugin or a conflict: deactivate all other plugins and switch to a standard theme (like 2014) for a moment and see if the error persists.
Hi there – sorry, forgot about the May post. It might well be the same thing. That’s a good point about debug.log, apologies for not looking there. What I see there is:
[22-Dec-2014 19:26:26] PHP Fatal error: Call to undefined method Mappress_Poi::get_custom() in /nas/wp/www/staging/chrisrae/wp-content/themes/kingtides/map_poi.php on line 8