On our LAN, several of the computers on it in the same workgroup, the problem is this:
Computer A can access computers B and C, computer B can access computers A and C, but computer C cannot access computers A or B. But this computer (C), can use the internet, therefor the LAN itself does work. When entering the workgroup an error message occurs:
"[Computer A] is not accessible. You might not have permission to use this network resource. Contact the administrator of this server to find out if you have access permissions."
But this is bogus. It was working fine just a while ago, and no changes were made to this computer by any administrator.