Start with quick checks: is the site URL right, are the database name, user,...

https://yonderstack.com/s/KDEA4-3Sd80TEpK7uCeOI

Start with quick checks: is the site URL right, are the database name, user, and password right, and is your host up? If so, try deeper fixes, like a database repair. Contact hosting support if the server is down or your login details fail again.