macbook pro 17inch late 2011does partial startup and then restarts

While using my MacBook this morning, the display suddenly went a bit odd with windows randomly broken across the screen. Then it went blank and rebooted. I signed in to my account and the small grey horizontal bar progressed until it reached about two thirds across then it stopped and rebooted again. But it gets no further than the two thirds across again and reboots.

I tried a boot up in safe mode - the horizontal bar gets almost all the way across, then the bar disappears and the grey screen remains blank. it goes no further and stayed like that for hours while I left it. I tried the hardware analysis (power on and hit the "D"key) but the symbol that's supposed to come up didn't. When I released the D key, it just started the above boot/reboot loop.

I'm running El Capitan and all other software is up to date. El Capitan has been installed for three to four weeks with no problems. I have installed no new software. I had very little running at the time of the problem: mail, aperture, safari (one window running Facebook) and preview.

Any suggestions and help would be appreciated.