Start by checking your database credentials, server status, and recent changes....

https://record-wiki.win/index.php/How_to_Tell_If_This_Is_a_Hosting_Outage_or_Your_WordPress_Install

Start by checking your database credentials, server status, and recent changes. If those quick checks fail, try deeper fixes like repairing tables or reviewing wp-config.php. Contact your host if the database server is down or you still can’t connect.