How do I fix my WordPress site when it’s stuck in “Briefly unavailable for scheduled maintenance”?

Persistent maintenance loop errors occur when a server update process terminates mid-execution, failing to automatically delete the hidden .maintenance file located in your site’s core server root file directory.

“This is a quick one, a leftover hidden file from an update that didn’t finish cleanly. I’ll remove it and have your site back online within 24 hours for $99.”
— Jeff Bollettino, Managing Director, Ignite Conversions