Q: Will not start properly after downloading El Captain

My computer will not boot after installing El Captain. It  starts up, gets an 8th of the way in the progress bar and then shut down. What do I do?

while El Captain was downloaded, it told me the copy could not be verified and that I had to renown load but for some reason I had to restart my computer. While it was trying to update, my computer went crazy, shutdown and restarted but it won't start and just shuts down. I don't know what to do.