So, this could be a ton of different things, but I'll throw some out there to
get you started:
1. Are the correct host headers entered in IIS?
2. Is IIS started?
3. Did you check the HOSTS file for errant entries?
4. You could try restarting the Windows SharePoint Services Web
Application Service in Central Admin, Services on server. This will remove and
re-add the relevant Sharepoint info to IIS.
5. If using assigned IPs, are they correct?
6. Was anything changed in 12Hive ?