‘Maintenance’ problem happens if you are updating WordPress core files, plugins or themes then you didn’t get a successful result shown on the summary after the update. Most common reason for this problem are some files might be corrupted, or a disruptions during the update which means that the files are not fully extracted or updated properly as shown on the image below.
To fix this, make sure you have the admin access – username and password to your server or hosting. You can your access your server or hosting via Cpanel under File Manager, SSH (for advanced user) or any FTP program. I’m using FileZilla FTP because the lightweight and free.
If you have more than one domain on your server/hosting make sure you access the right path for your WordPress website that’s having a problem, under its directory such as “public_html/” structure you will find a “.maintenance” file and you have to delete that file like the image shown below, then refresh your WordPress website again. Now make sure you run the update again so all files will be extracted or updated properly.