My Macbook Pro is taking a very very very long time to boot up.

I have a mid 2012 Macbook Pro that is taking a very long time to boot up. I updated the OS a few weeks ago and haven't had this problem until today. The apple screen came up as usual and the bar began to move under it until it got to the end and its been like this for a couple of hours. I've turned it off and restarted with the same result. Any ideas of what to do from here?

Thanks,

Billy