At times WordPress might break down, or even break down completely. For example, accessing any administration pages might give a 404. The causes are numerous but the best way to start the process of isolation is to disable all plugins.
There are two ways to disable all plugins:
- Rename the plugin folders one by one.
- Use the database query below to disable all plugins in the database. Please note that all auto settings will also be removed.
mysql> update wp_options set option_value = 'a:0:{}' where option_name = 'active_plugins';
Reference
https://wordpress.org/support/topic/wp-admin-page-is-not-found/