I use Mandrake 9.0 and Libranet and the latter has worked fine until this morning, but now every time I try to log in, I get a popup that says "Your home directory is listed as /home/myusername but it does not appear to exist. Do you want to log in with the root dir as your home dir? It is unlikely anything will work unless you use a failsafe option". I have googled this error message or parts of it and found absolutely nothing that seems very relevant or helpful. The only thing that is different about Libranet from the last time I booted it is I copied over some directories from my Mandrake partition into my home directory (/home/jmelton) -- the one that the login screen error message says doesn't exist!

But I get the same error message regardless of which user I log in as, except root. I even tried creating a new user and then trying to log in as that user, and got the same error message. Anyone have any idea what I should do, short of reinstalling Libranet?[/i]

Welcome to LQ.

It sounds lie a permissions problem. Can you post the output of:
ls -la /home

ls -al /home shows:

drwxrwxr-x 6 root jmelton 120 Nov 30 12:35 .
drwxr-xr-x 22 root root 504 Nov 30 12:19 ..
drwxr-xr-x 11 jeff jeff 576 Nov 30 12:37 jeff
drwxr-xr-x 30 jmelton jmelton 1480 Nov 30 12:32 jmelton
drwxr-xr-x 12 root root 632 Oct 20 04:19 rebecca

I actually suspected that it might be a permissions problem and have been fiddling around with that some. Before, for some reason, /home itself had 501 listed for both owner and group. I have no idea why; every other main directory was owned by root, which is the way it is in my Mandrake distribution.
Also curious is that user rebecca is owned by root instead of herself, but that isn't the problem here since none of the users can log in.

Try this:
chown root:root /home
chown -R jmelton:jmelton /home/jmelton

Then try to login as the user "jmelton".