You have had a recurring problem in which, after running a task (high workload?) for a long time (system update, streaming multimedia) your system would execute a shutdown. It always rebooted fine so you thought nothing of it (after all, you are used to frequent, unexplained, system crashes from other OSs you have used?)
This time the system won't reboot.
Before I get to your problem... your symptoms would have been very worrying to any other linux user. Linux is very stable
. In some OSs this stuff is just business as usual, but in linux unexplained shutdowns can preclude a fatal system crash and can be a sign of dying hardware. There are logs to help troubleshoot.
In your case: do you remember if your last update involved a new kernel?
What you need to do, in the first instance, is see if you can boot to a previous kernel or rescue mode. You do this from the grub menu. Get back to us...
Alternatively, boot the live CD, and check the filesystem on the HDD.
RAMDISK: ran out of compressed data
Hopefully you just had a partial upgrade before the crash. Thus, a partial ramdisk, thus, no boot.