some sites on my multisite network have stopped responding.

I have recently installed domain mapping, and had trouble getting one of my sites to respond.
I then made changes to the force settings, took if off HTTPS. Then it seemed to resolve fine. I have been working on the site using upfront this afternoon, and now it seems not to respond again. In addition 3 other sites are unresponsive.,, and Can you help me figure out what I may have done wrong, or what i need to do to fix it?