Depending on your server configuration you can run into memory limits that prevents your site from fully working. This can happen because you have a website with many plugins or complex theme that uses a lot of memory. Even simple plugins can bring big database queries to the website if they try to process data and execute code at the same time.
This is usually due to a limit imposed by the hosting company. You can either try to edit a file on your server of if that does not work you can contact your web host to increase the limits.
Edit wp-config.php
On some servers you are able to change the memory limit via your wp-config.php file, if the following does not work you will need to contact your web host to make sure your website has enough resources.
Edit your wp-config.php file, either edit the value if WP_MEMORY_LIMIT is already defined or copy-paste this block:
define('WP_MEMORY_LIMIT', '1024M');
ini_set('memory_limit', '1024M');
Resolve conflicts with other plugins
As previously mentioned this problem can also occur in combination with other plugins on your website that uses a lot of memory. Turn on WP_DEBUG and use the debug.log file to help pinpoint where the conflict is. Use WP_DEBUG to solve problems.