error establishing a database connection wordpress

Hello,
Last days, I bougth 2 domains on OVH (sama-slodycz.pl and alkotfilms.com).

The goal of it was to create wordpress websites.

When the first one was nearly finish, I buy the alkotfilms.com one. I didn't know so well how it worked, so I first created a new "Hosting Plan" in the web cloud for this new website. But a payment was needed from there to create a wordpress website with my new domain name.

In the package that I buy first, it was written that I can build up to 10 websites, so I tried to add the alkotfilms.com domain to the first Hosting plan that I have created and related to sama-slodycz.pl.

KIn "Hosting Plans", I went to "multisite" and tried to add alkotfilms in it. But it request me the password of the database, the password was not working. I changed the cart, select "database", changed password, and since then I have this error message "error establishing a database connection". This appears for both websites.

I checked tutos on Youtube, but I'm not a programmer. If that is the only solution, I don't know how to change the wp config file.

I will appreaciate your help, because I'm starting to desesperate.