frozen MacPro startup

While working online my MacBook Pro's screen suddenly went solid grey. I powered it off by holding the power button and then started it back up. It would load the apple start up bar and after it got to about 1/3 of the way suddenly turned off. I tried this a couple of more times, and then opened it in safe mode. Ran the disk repair and after receiving the message in green 'everything appears okay' the screen froze. It said there was 5 minutes to go and I waited at least 20 minutes. I then tapped my mouse and the rainbow ball started circling and I waited longer until I powered it down again. I tried starting it up again and got a flashing question mark inside a folder. Powered it off and started it up in safe mode this time the bar starts but gets stuck half way and won't move. Any suggestions what I can do? Thanks for your time!