my Macbook pro is not turning all the way on. why?

when i turn my macbook pro on the apple logo shows up with the white background. but then a little gear pops up below the apple logo and starts spinning around for a couple of seconds. it keeps doing that, when this bar pops up below the gear and it starts loading something. it does that for like 5 to 10 minutes and then powers off it does that every time. what is the solution. it won't turn on all the way.