Progress bar stuck during update installation

My 13" 2013 macbook Pro is stuck. I decided to upgrade from Yosemite to El Capitan, so I downloaded it and started the install, but it has gotten stuck. The progress bar is only a little bit of the way full and it says "Installing: about 31 minutes remaining," and the number has fluctuated between 28 and 31. However, the progress bar has been in the same place for a couple hours now and I'm getting worried. I tried rebooting it after the same thing happened earlier. Any ideas what I can do? for what it's worth, its been running unusually slow for the last week after shutting off due to overheating. Thanks!