You’re right in the middle of updating your install of WordPress to the latest version when suddenly there’s a solar flare! — and everything stops.
When you reload the Updates page, instead of the expected blue “Update Now” button, there are these words:
Another update is already in progress.
You check the front end but the site still accessible, so you know you’re not stuck in maintenance mode. You check wp-content/ and find an upgrade/ directory there, so you remove it and check again — no dice.
When WordPress initiates a core update, it adds a record to the options table — 'core_updater' before WP 4.5 and 'core_updater.lock' for WP 4.5 and after — in order to prevent multiple core updates.
If you have WP-CLI installed on your hosting environment, you can remove the lock with this command:
wp option delete core_updater.lock[Note: This command uses the WP 4.5+ option name. If you’re running an older version of WordPress, you’d use core_updater instead.]
And if you don’t have WP-CLI, just wait 15 minutes — WordPress will remove the lock automatically.