Downloading this update seems to have killed my powerbook. Any ideas?

Downloaded update as per normal. Dialogue box to restart, so I pressed OK.
It closed down apps and then restarted, but a screen I've never seen before
came up. 
It has a large dark grey box in the middle with a large icon of the power
button. It reads: You need to restart your computer etc etc

On top of this, across the screen, is a lot of numbers and words mentioning
things like Kernel version - No debugger configured - dumping debug
information - exception state and so on. It ends with "panic: We are hanging
here...."

Have turned the machine off many times via the Power button. Have restarted
holding down Control-Command - same thing.
Have tried restarting then holding down Option-Command and R and P. Let go
after the double restart - same thing.

Thanks for any help before I call the emergency services.

Matt.