I think you should check your code again or traffic.
Database down because low of ram.
Low ram because error code or high traffic.

You will need to repair your database and check for errors among a plethora of different possible causes. It’s way too long to type and explain so I will share an article here: https://mythemeshop.com/blog/fix-the-error-establishing-database-connection-in-wordpress/ for such a case. I hope the admins realise I’m not trying to spam. Just trying to be helpful.