System crash - now shows "no space on device"

[joe at actionline.com]

My system crashed today - screen just went black when I clicked on the
calendar.  Could no longer boot to kde.  So, I booted into safe mode and
'df' shows the root partition (12 gig) is filled.

My home directory seems to be intact (thank God), but I can't figure out
what has happened to fill up the root partition.

'du' on '/' shows a total of only 3.5 gig of space used, so why is the
rest of the root partition filled up with 12-gig of space used?

I tried to ssh login to the system from another computer wire-connected on
the same ethernet but 'ssh' says "no route to host."  I did
'/etc/init.d/sshd start' and got the response "OK" but I cannot connect. 
'ifconfig' is found on the system, but returns no response when I invoke
it.