Macbook pro won't load past startup screen and grey loading bar keeps occurring

Midway through using my macbook pro the system shut down and now continues to reboot itself but would only load as far as the apple screen with the grey apple logo

. This happened several times then it progressed to the point in which a grey loading bar appeared, at first the mac kept shutting down when the bar was midway through loading, now the bar fully loads but the system still shuts down.

Any ideas what may be happening?