I have some domains through aplus.net, yestoday I changed my name server to another host, the website cannot show up, I try to ping the website, it shows:" Ping request could not find hot www.mydomain.com, check name and try again". You think this problem is from host or from domain registration side? The name server is wrong?

I try to addon the domain from control panel:

The subdomain, myusername.mydomain.com has been added.

Nameserver ips for mydomain.com are: 11.11.11.11,11.11.11.12 Sorry, that domain (mydomain.com) is already pointed at an ip address that does not appear to use dns servers on this server or master servers! Please transfer the domain to this server's nameservers or have your admin add one of its nameservers ips to /etc/ips.remotedns and make the proper A entries on that remote nameserver!

Removed Entry from httpd.conf The subdomain, myusername.mydomain.com has been removed.

mydomain.com could not be setup. The subdomain myusername.mydomain.com was not setup either.

How to solve this problem?

check it from www.dnsreport.com it can tell you few things for fixing stuff here and there

FAIL Lame nameservers ERROR: You have one or more lame nameservers. These are nameservers that do NOT answer authoritatively for your domain. This is bad; for example, these nameservers may never get updated. The following nameservers are lame:
66.79.180.131

so ask your host to fix that lame nameserver :D