my macbook air continuously turns itself off and on again for no apparent reason- despite having full battery... What is wrong with it?

As of recently my macbook air continuously turns on and off.

Sometimes it does not turn on at all. When it does turn on, after a few minutes it comes up with the shut down options and unless I press cancel it turns off. Occasionally when turning on it only gets as far as the startup sound then turns off again- occasionally in a continuous loop.

Any ideas?

HELP!