an awful problem, disc space is disappearing

[Gabriel M Dragffy]

This morning my root partition was 6gb large and had 49% free space.
This evening I was trying to install the game supertux and found there
was no disc space. I'm using a simple desktop installation. I have
apache and proftp installed and running but a firewall blocks remote
connections to them. Something just ate about 3gb in one day and I
didn't do anything to warrant that. Furthermore only yesterday I had the
same problem but I put it down to a power failure, I ended up deleting
all my LVs, my PV, VG and the partition and restoring.  How do I find
out where my 3GB went? And then find what took it and why?