My macbook air keeps shutting down during startup.....help!

Hi. My macbook air (new in Nov and worked perfectly so far) keep shutting down during the start-up process. It powers up fine, but then as it is booting up I get the apple sign and the grey screen and then the line loading across the bottom.....and it gets about 1/4 of the way across then it shuts down.

I have triend the SMC and PRAM reboots but same thing hapens. Just when it looks like it is going to work, it shuts itself down.

Very upsetting! Can anyone help me? Thank you!