Multi-site User Messed Up Their Domain Name! Please Help!!

Hello,
One of my multi-site users made a mistake and clicked on the domain mapping feature and changed their default domain that they signed up for (nubi.nuwshop.com) to a domain name that is not even active (puzzlerockingchairs) -__-
So basically, now when I type in their nubi.nuwshop.com"website I get directed to a blank "could not find this page" screen.
I can't even go into their backend to correct it because their entire nuwshop domain has been redirected to a blank page!
How can I fix this? I need to remove what they entered in the domain mapping area and make the correct nuwshop domain appear.

The good news is that from the homepage, I can still see the products they posted up for sale, but I can't get to their website.

PLEASE HELP!

Thank you way in advance!!
Jafar