When your server reaches maximum usage and uses up all the server resources, this error usually pops up. The problem could be hidden in one of the themes and plugins that you are using.
Solution
It is advisable to start switching to default theme and see if the problem persists. If the problem continues, try to deactivate all plugins and activate one plugin after another to detect which plugin is resulting in this resource utilisation. If you detect the plugin that is causing this problem, there are chances that this plugin uses more memory resources from your hosting server.
The ideal way to fix this issue is to first consider if the plugin in question is worth using all the resources – meaning, is it coded properly enough to justify the usage of the server resources. If yes, then you can ask your hosting provider to increase the memory limit or the memory execution time. If you already have the access to php.ini, then you can increase the memory execution time on your own using this code:
max_execution_time = 60;
Alternatively, you can also edit the .htaccess file and add this line:
php_value max_execution_time 60
If you edit .htaccess, make sure you update your permalinks by going to Settings in WordPress dashboard.