My blog suddenly disappeared and says "Error establishing a database connection"

My company had a blog that we posted to once a month. The last time we posted was at the end of May, and everything was fine then. We went on to post at the end of June, and all of a sudden, the blog has disappeared.

The URL is smartworkout.net/blog
When I type the URL in and hit enter, absolutely nothing happens. The browser stays on the current page.

However, when I go to our website, https://www.smartworkout.net, and click on our link to the blog (the orange icon with the “B” on the left), it takes me to the page, but it is blank and only this error shows up on the screen: “Error establishing a database connection”

I tried to log in using the information I always used for the admin login. At first, wordpress was telling me that my username did not exist. I tried again a few minutes later, and it asked me to reset the password and then logged me in to a blog that has not been updated since 2010, with the URL smartworkout.wordpress.com. This was the blog that we had before we linked it with our website, smartworkout.net, and changed the URL to smartworkout.net/blog. Our webmaster had done this for us but we are no longer in contact with him so I am not able to ask him for advice.

Please tell me what could be causing this and what I can do to fix it. Thanks in advance.