Machine does not boot and has rapidly blinking sleep light. What does it means?

Last night I left my MBP open, turned on and unplugged. Safari was the only application running. Usually the MBP will fall "asleep" in approx. half hour (when running on battery). This morning when I tried to use it, the machine did not respond to keyboard/trackpad inputs. I thought the battery had drained and plugged it to boot it up but the sleep light started blinking fast, the MagSafe indicator lit (and stayed) green, and there was a sound from (what I suspect was) the DVD drive as if it was trying to spin but couldn't.

The machine did not respond to the power button, or keyboard / trackpad inputs. When the battery status button is pressed, the first indicator light flashes rapidly for a few seconds and turns off.

Unibody MacBook Pro 15in i5 2.4GHz 4GB RAM 320GB HDD

Model: MC371LL/A

OS: SL (up to date)

Other facts about my Mac

- Had it for 18 months

- Has never been opened. All internal components are factory originals.

- Has never been dropped.

- Peripherals: CanoScan Lide 210 (>1 year), Samsung ML-2165W Wifi printer (< 3 months).

- Use it mostly with a belkin cool mat

- To move it from one place to another I put it to sleep and wait for the sleep light to start blinking.

- I shut it down at least once a week.