Let's suppose you updated -current >= March 1 2010 and have a nVidia GPU
Question: the screen goes blank during startup what can I do ?
Answer: reboot typing nomodeset at the Lilo prompt, like this:
Explanation: for nVidia users and kernel 2.6.33 or newer, a nouveau driver is automatically loaded during the startup sequence. But if you chose a VESA resolution at boot like most people do (and is chosen by default if you don't edit lilo.conf before running lilo), the nouveau framebuffer doesn't work for now with some GPUs including mine, and vesa framebuffer is no more in use=> no display at all on the console.
But if you use the nomodeset kernel parameter, nouveaufb is not used so you keep vesafb and all goes well.
Once you are given access to you system again, better either blacklist nouveau or edit lilo.conf to choose one of the "VGA=normal", "VGA=ext" or "VGA=ask" option in your lilo.conf. This way you won't encounter this problem any more.