Just a grey screen after updating and restarting

Just tried to update MacBook Pro mountain lion, to the latest software. Seemed to go okay until I re-started it, now all I'm getting is a grey screen and my mouse. That surely shouldn't be the case, plus has been like this for a good half an hour. It said do not turn off, I take it that this would damage the laptop and I don't want that to happen. Please help, and anything will be good