Why won't my MacBook Pro switch on?

Last night, my Macbook Pro (Early 2011 Edition) switched off suddenly and will now not turn back on.

If I press the power button on it's own, nothing happens. If I reset the CRM with ctrl-shift-option, the power kicks in for around 3-5 seconds, before cutting out.

When this happens, the fan sound is often louder than usual.

If I hold down the keys to reset the PRAM, when I get it switched on, the Mac will stay on until I let go of these keys and then switch off almost immediately.

The magsafe light is at green when plugged in. I have had my Macbook for 3 and a half years.

Can anyone point me in the right direction, before I have to take it to Apple and spend lots of money.