Cannot gain Web access to server

When I left the office on Friday, our websites were all working properly. When I came into the office this morning, I couldn't gain web access to any of them.

I can SSH into the server. I can restart apache, which happens with no warnings or errors. I have SFTP access to the server.

I'm not sure what to troubleshoot. There have been no DNS changes and my zones still check out okay.

Any thoughts on what to check? There are no errors in Apache's error.log worth noting, just a warning about a misplaced directory for a site that I fixed some time ago.