When I ping one of my domains, I get the error message "Unknown host". What is causing this problem? Is it related to the name server?

Could be DNS down, server down, who knows, fire off email to host (maybe try to ping from another location first).

Probably it is name server error. It would be easy to understand the real problem if you can post the domain name.

The domain name is nvbrothels.net . Pings fail from all locations I have tried. My hosting company has not responded to my emails yet.

Looks like you were hosting your site with version12.net which was recently sold.

Read this thread: http://www.webhostingtalk.com/showthread.php?s=&threadid=15677&highlight=version12

Yes, I am aware of the sale. There was a little under a day of downtime when they moved the servers to the new location, a couple weeks ago. However, the move is complete. I am not aware of any connection between the new ownership and my "host unknown" problems.

The DNS servers for your domain are listed as:

NS1.VERSION12.NET 63.80.246.7
AUTH110.NS.UU.NET 198.6.1.114

However, neither contains information for your domain. So your domain does not resolve.

You need to get in touch with 5 Start to resolve this issue. Their website is http://www.5sc.net

I have contacted support, as well as several inidividuals. I hope they monitor emails 24/7. If there is a problem with the name server, my IP address should still work. Right?

My IP address is 63.80.246.182 . Ping's to this address have 100% packet loss.

What might be causing this problem?

I hope they did not delete my web site. It contains a years worth of work. I am concerned that I went well over my bandwidth quota last month, and the problems occurred on the final day of July, just after reading the bandwidth totals (my throttle-me total is done on a 30 day/month basis).

Yeah your IP is not resolving too. That means the IP address is not binded to the server on which your sites is located. I don't think that they have lost your data/files. It could be a simple mistake (missed to bind your IP address).

Note: Always make sure you have a back up of your files/data on your local computer.

I just heard back from support. Apparently they removed my site. Version12 allows sites with adult content, but the company that bought Version12 does not. I think they sent an email out to some of the sites with adult content before they removed them, but they did not send an email to me. The support people told me they will try to have my account online later today, so I can download my files. It sounds like they have not deleted them yet.