G4: After security update, won't boot! WTF

Try one of these ideas from Apple:

My Mac displays a blinking icon at startup
Sometimes when you try to start up your computer, your Mac may show you nothing but a blinking question mark or a blinking globe icon. This is your computer's way of telling you that it can't find the system software it needs to start up.

Sometimes the blinking icon is momentary, then your computer starts up normally. Other times, your Mac won't finish starting up at all. Usually, all you need to do is remind it where its system software is. Here's how to resolve both scenarios.


My Mac won't start up
Insert the Software Install disc that came with your Mac.
Restart the computer and immediately press and hold the C key to start up from the disc.
When you see the Installer screen, choose Open Disk Utility from the Installer menu.
Select your hard disk volume in the left pane, click the First Aid tab, and then click Repair Disk.
When it's finished, restart your computer.
If you still see a blinking question mark or globe, try this:

Restart your computer and immediately press and hold the X key.
Your coumputer will start up to a gray screen and then restart again.
Let go of the X key after your Mac starts up the second time.
If that doesn't work, try resetting your computer's PRAM (parameter random access memory), then repeating the X key steps above afterwards.

Restart your computer and immediately press and hold the Command, Option, P, and R keys simultaneously before the gray screen appears.
Continue holding these four keys until you hear the startup sound for the second time.
Release the keys to allow your computer to start up normally.
If you still get a blinking question mark or globe icon, call in the cavalry.