Help.... I clicked upgrade to the new OS X Yosemite about 3 hours ago - now all I have is a black screen with the apple logo and a progress bar that hasn't moved past about 1/3 of the bar - I tried to shut the iMac off but it won't do anything.... What should I do? Have a test online to do this evening 
hope someone can help with what to do... Thanks
hope someone can help with what to do... Thanks