I had this issue today morning while installing 10.10.3 and this is was I did.

1) I forced shut down the mac by pressing and holding the power button.
2) I booted the machine in recovery mode. (for those who dont know, you have to press and hold the command+R buttons and then press and hold the power button till your machine starts OS X: About OS X Recovery - Apple Support)
3) Once the machine booted in the recover mode, You'll be presented with options like restore from time machine, reinstall OSX, etc.

I didn't find the right solution from the Internet.
References:https://discussions.apple.com/thread/5565827?tstart=0

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