After moving WordPress multisite site from one hosting to another I have encountered problem. One of blogs didn’t open. It was displaying “Error Establishing a Database Connection”. It was a little bit confusing because I was sure that wp-config has proper database settings and other sites worked. After several hours of searching, googling, trying to change db settings I found that one of sites is missin in _domain_mapping wp table. And that was it. Just needed to add missing site and evertyhing worked.