Why does Yosemite hang on restart after power loss?

When Yosemite reboots after an unexpected power loss, on my iMac, the start up screen hangs with the progress bar half way across. The only solution I have so far is to re-install Yosemite. Turn iMac on and then on again after a few minutes ends with the same result.

I had a similar experience, my installation never rebooted itself. After hours spent with Apple support I finally resorted using the Verbose command on restart (command key & V) held down before the grey screen appears. That will at least give you a clue where in the boot process restart is failing. My own adventure found the install HD had errors that Apples disk utility never identified. Hope your experience is better.

We use Active Directory on our network. iMacs bound to Active Directory and running Yosemite do not recover from an unexpected power loss. The Macs hang on the Apple start up screen with the progress bar about half way. The Macs with Yosemite had no issues running in Active Directory until this problem.