Raspberry Pi Won't Boot, Red Power Light Only

Yesterday one of my Pi worked perfectly, but today just the red power light turns on. There is nothing through either HDMI or Composite outputs and the OK light bearly turns on.

Yesterday it worked with a 32GB Toshiba SD card, Logitech wireless USB keyboard, , Amazon 5V 1.8A charger for a Kindle.

I tried re-flashing the SD card and tried a Lexar 4GB SDHC Class 2 card and neither worked. Unplugging everything but the power and SD card didn't work either.

Reading about this problem on various forums it seems to be quite common so I will need to play about with the raspberry Pi and try and find out what if if I'm able to bring it back to life