Can no longer access computer on network - Windows XP?

We have a computer that we can no longer connect to across the network. My account is an administrator on the computer and trying to access anything at all on the computer does not work. \\computername brings nothing up, \\computername\data (a shared directory) doesn't work, and \\computername\c$ doesn't work. Last week, all of these worked.

No new software has been installed, the computer is on the network and can be pinged just fine, the firewall is disabled, file sharing is enabled, like I said, it worked last week and had for over a year before that. We have restarted the computer many times.

I've tried creating a new share as well but, of course since I can't access it at all with \\computername, that doesn't work either.

Anyone have any additional ideas that I can try?

Thanks!