A file named .maintenance is placed by WordPress during an automatic upgrade to notify visitors that the site in under a short maintenance. If an upgrade fails or is corrupted, the file will remain in the WordPress folder and won’t get deleted.
To remove .maintenance file,
Access your blog root directory via FTP or filemanager.
Locate the file .maintenance.
remove the .maintenance file from the blog root directory.
Once you solve this problem, you may want to run the WordPress automatic upgrade again.
