MacBook Pro won't start

I have a late 2012 MacBook Pro running on High Sierra. It has been working fine until the most recent (Feb 2018) security update. The update installed and I was using my laptop for a few days after that just fine. However, during that time I did not shut it down or restart it. I went out of town for the weekend and shut down the laptop (did not take it with me). When I came back to restart it, I could log into the laptop, the progress bar would continue to the end (took a longer time than usual) and then stop, stuck there. I've tried this a couple more times but never got the laptop to open. I would log in, the progress bar would run to the end and then stop. I just let it continue like that for at least an hour before giving up. What can I do to fix this?