I have a 13inch MacBook Pro and when I switch it on the apple logo appears then a slot and the circular symbol which is turning and then before opening the home screen it switches off again. Is there a sequence that I can use to force it to load.

I have got a 13 inch MacBook Pro and it got stuck in a loop so I switched it off by holding the power button down and now when I switch it on the apple logo appears then I get a blank slot at the bottom of the screen then the circle appears and rotates and then it switches off again, I have tried starting it again with the system disc that came with it when it was new but it makes no difference. Is there a sequence that I can use to force it to start.