Start with quick checks: verify database details and see if the host is down....

https://www.blaze-bookmarks.win/your-wordpress-site-is-down-start-with-quick-checks-verify-the-database-name

Start with quick checks: verify database details and see if the host is down. If not, move to deeper fixes like repairing the database and checking site files. Contact hosting support if the database stays unreachable or you lack server access.