There are many things that can cause problems with your WordPress website and in turn can cause fatal or critical errors that can stop you from viewing your website and even logging in to the WordPress admin area.
These can be caused by anything from a conflict with a theme or plugin to a htaccess file issue and many things in between.
The fastest way to try and find what could be causing the problem is to view the error log. This is located in the root directory of your WordPress install and should provide you with valuable information on what the issue is. For example, if you notice error messages regarding a particular plugin, it is likely there is a conflict with that plugin. In this case, go to the wp-content folder and then plugins and rename the plugin folder name to disable it.
Then try accessing your website again to see if this has solved the problems.