Accidentally installed WordPress on my existing website again, now my site is gone. What do I do?

I have two domains running on my Server and an existing website running on Domain A. Last night I wanted to download WordPress for Domain B, but my sleep deprived brain selected the wrong domain during the process and reinstalled WordPress on Domain A. Now my Website is showing one of those basic Hello World Templates. Luckily, I did do a backup beforehand but I am not sure how to get it up running. Also, the original data base is still on my Webserver, just not showing on the website where it is supposed to be. Any helpful advice is welcome!