Error Establishing a Database Connection

I’ve not edited anything on my website for a while and suddenly see the message “Error Establishing a Database Connection” appear when I try to connect.

https://www.marcbally.ch
My hosting provider is EasyGiga.com and I use Plesk 9.5.4 to manage the domain.

Apparently, this is a WP issue, because there’s a part of the website which is not managed by WP that is online and ok: https://www.marcbally.ch/index2.html

I’ve been having some issues while installing and posting, but since I didn’t even connect to my database for days, I’m surprised to discover this sudden error.
I can’t even access the /wp-login.php page.

Any advice about what could cause this?
Thanks in advance!