MacBook 2008 suddenly turns off - why is this happening?

I have a MacBook 2008 and recently of has started just suddenly turning off. I will turn it back on and it dies within 5 minutes. Is this likely to be a battery issue, or could it be something else? A month before it had a kernel panic & I had the hard drive repaired at the apple store. Not sure that is relevant but thought it might be? Thanks!