I have a macbook air when I went to start it I got the prohibitory sign. I powered down started it up after about a minute and held down the option button once I heard the chime. I got an icon for the macintosh hd and a window. I clicked on the arrow under the icon and it started to load. half way through it stopped and went back to the prohibitory sign. This happened five or six times. any suggestion on what to do next?