Faulty xorg.conf caused X to disappear!
I wasn't able to change the resolution in X so finally i tried the xorg.conf-vesa template file. It booted X, but during Gnome loading the screen suddenly added a couple of distorted lines over the Gnome logo, froze completely and there was nothing to do but turning the machine off.
Now, X is gone. Yes - startx gives "no such command". The X11 folder is RED when ls:ing the /usr dir and cd X11 gives "input/output error". ... this ... this can't be?
If this is a hardware error, why did only one single folder disappear - and the X11 of all possible! The installation seems intact in all other ways.
The computer is a 200mhz 64mb ram 4gb hd and some 1mb gfx card (quite a relic, but fully working). I'm running Slackware 10.