This is where the strangeness started. I was able to boot it back,
but then it
keeps rebooting after the boot process is done. It would get to the state
where I need to log in, and within several second, it would reboot itself.
This happens over and over.
First thing I would do is boot the live CD image, and then from the live
CD image mount your real file systems and make a back up of everything
important.
Also see how stable the box i running the live CD.
If it's stable running the live CD I would take a hard look at your real
disk with chkrootkit etc and see its rebooting due to a misinstalled
trojan or similar, especially if it was being reliable before. It could
also be a failed update, or a particularly unfortunate disk corruption.
Clearly also try booting older kernels off the boot menu.
If it isn't stable running the live CD you may have a hardware problem,
or some modern laptop hardware has been known to get itself into a state
where it stays hosed until power is physically lost. On some Ultrabooks
that's a nightmare as the battery isn't removable, but on most laptops
removing the battery for 10 minutes does the trick.
If it still isn't stable you may have to look for hardware faults such
as a fan failure.
Alan