It's a problem with our main IIS server. The forum and chat servers are working fine.
I'm still trying to find where the problem is -- but I think it's either a bad disk sector in the RAID array or a problem with our MSSQL Server. Hope to have it fixed soon.
In the mean time, the main server(www.studentdoctor.net) may be offline intermittently. So, be sure to bookmark the forum server: forums.studentdoctor.net