Why is my website often put in maintenance mode by itself?
Usually, a Drupal site enters in maintenance mode by itself because is having problems to access to its database or when is set on that mode through the option located at "admin/config/development/maintenance".

If you are not able to log in on the site, that could indicate Db problems.

Check...:


at "admin/config/development/maintenance" is checked the option "Put site into maintenance mode". Or with drush: drush vget maintenance_mode 
the settings.php if the Drupal database parameters are ok.
the status of the Drupal database, (if the user:password are correct, if the database server is online).
the error logs (MySQL, PHP, Drupal).