WARNING:
You must not send PM to Tech Support members asking if you are banned.
You can ask it here, in the chat or opening a thread here in Tech Support ONLY if you already tried all the suggestion below.
Things you can try to do are:
1. If you are using Skidrow client or another different from aIW, download the original aIW client. If you already got this, go to 1b.
1b. Go to alterIWnet.ini in your modern warfare 2 folder and change server=server.alteriw.net to server=109.237.208.88
(If it was already 109.237.208.88, then change it to server.alteriw.net)
remember: after that, open Task Manager, click File, than Run and type ipconfig /flushdns
1c. Always in alterIWnet.ini, make sure that "Webhost" is set to "auto"
(thanks to Ficiani)
remember: after that, open Task Manager, click File, than Run and type ipconfig /flushdns
2. go to your players folder in;
C:\Program Files\Activision\Modern Warfare 2\players (or the steam one)
and delete these files;
config_mp.cfg
settings_c.zip.iw4
settings_m.zip.iw4
3. probably, if it doesn't still work you have been banned.
If you are a cheater, don't even try asking a aIW member to be unbanned.
If you aren't a cheater, maybe you have the same XUID of a cheater, or a similar nickname, or the same IP (quite impossible, but maybe you have been in the same Internet Cafe where was a cheater).
So go to aIW chat and send a PM to one of the people in the list who have the aIW tag, or ask in the main chat.
Follow aIW instructions and your error will disappear.