Macbook doesn't boot up

Today all of a sudden my Macbook refuses to boot up. When it starts-up, it goes to the gray screen with the apple logo on it (and that spinning wheel) but doesn't continue. I've left it like that for hours and it still hasn't loaded the login screen. I can hear the HD working...so don't know what the problem is. I've tried rebooting a number of times but still the same problem every time. Please help!!!!!