Small Joomla teaching lesson needed for password(s) reset (German language preferred)

Dear Freelancers, I’m looking for some teaching assistance about Joomla. I run at the moment three different pages with the same provider where I can change all my passwords (FTP and MySQL). After I did that one time, all my pages where down and I see only a white page where stands something like no database or something like that. Luckily the provider helps me out of this situation and reset the old password, but he told me that I have to change also something in my three Joomla pages if I want to do that, but I don’t know where and how. I try the [login to view URL], but maybe I made a mistake or it was not all. As I had many different people working on the three pages, I think it would be great to change that. Who can write that down for me with some screenshots, how to do that. But please in easy words for “stupid users”. Thank you