Solution: Remove .maintenance file
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.