There have been a handful of bugs that cause problems using host headers. You had it working for a while, so at one point you had a compatible patch level. It's possible that a software upgrade, patch installation or other configuration change caused a system file to be replaced with a flawed version. So, it may be worthwhile to reinstall the latest service pack and hotfixes on your IIS system.
You obviously know how to configure host headers, because you had it working. It's worthwhile to verify that both sites are configured to communicate on port 80 and have the correct hostnames. Once you verify the server configuration, double check that the hostname being entered into the user's browser exactly matches the hostname entered into Internet Services Manager.
This was first published in November 2002