When WordPress shows “Error establishing a database connection,” start with...

https://felixssuperperspective.brightsora.com/posts/what-is-a-qps-charge-sheet-and-how-do-i-read-it

When WordPress shows “Error establishing a database connection,” start with quick checks for your site address, database details, and recent changes. If those look right, move on to deeper fixes like repairing the database