upgrade hangs

Trying to upgrade my macbook pro from 10.8.5 to Yosemite. The update process seems to go through but then when it says about a minute remaining nothing happens anymore. I waited over an hour to no avail.

I shut down the computer after that waiting time and restarted it. It just out of itself then restarted the installation process but again after about 50 minutes of normal progress … "about one minute remaining" and it hangs again.

What should I do?

HELP!!!!!