Can't connect to cifs server (but it used to work!)

Hi,

I have a Network Attached Storage device on my network - which is
basically a simple Linux machine used as a file server. I've
configured it to share a volume using cifs. All other protocols that
it supports are disabled.

I've got three computers on my network - one Windows XP Professional,
one Windows Vista Home Premium, one Mac. When I first set up the NAS
device several months ago, they all could see it and use it just fine.
The only anomoly was that Vista wouldn't remember the password even
though I kept checking the save password box, but this was a minor
annoyance.

Now, all of a sudden, Vista can't see the NAS disk at all. Doesn't
show up anywhere in the network explorer, and if I try to attach
directly to it with "\\servername\volumename" I get "The system cannot
find the file specified". However, I can ping the server and access
its web-based administration console, so I know it's out there and
reachable.

I didn't do anything to cause this. I haven't touched the NAS
configuration in months, and my other machines still see it just fine.
I tried turning off the Vista firewall and even the UAC, but it made
no difference. I found some articles on the internet that advised me
to change my LmCompatibilityLevel in the registry from 3 to 1, and I
tried that too, to no avail.

What could possibly be going on here?

Thanks,
Frank