Macbook pro does not display anything

Model is Macbook Pro 15" Mid 2009 Unibody

Mountain Lion

Modifications:

CD drive replaced with additional 500 GB Hard Drive

One of the 2 GB RAM chips replaced with a 4 GB chip

Earlier today I was browsing the internet on my MBP and suddenly the laptop froze, the screen froze, the mouse and keyboard were not responding at all, which eventually led me to have to force shut it down (by holding the power button). When I tried to turn the laptop back on it started up (as in I could hear the fan and the white light came on) but there was no display and then 4-5 seconds after it turned off and then started up again by itself and stayed on, still no display on the screen at all, its off and the keyboard doesn't seem to be responding either which I figured by trying to press the caps lock button to see if the green light comes on in which it didn't.

I have tried pressing the Ctrl+Shift+Alt+Power button and then turning the laptop on but the same problem still persists. The laptop starts up, turns off and then starts up again and then stays on with nothing displayed on the screen and no response from the keyboard. The start up chime also doesn't occur. I am unable to start the laptop in Safe mode or any other mode that requires me to hold a button when powering up as the laptop turns off after 4-5 seconds and then starts up again. I have looked online to find anyone with a similar problem but can't seem to find any posts regarding this.

The laptop has not been dropped or anything, neither had it experienced any liquid damage. It has been running smoothly before the problem that occured earlier. I am in desperate need of my Macbook Pro and there doesn't seem to be any Genius Bar appointments available in the next 7 days in any Apple store near to me (Central London).

Any help or advise will greatly appreciated.

Khal