Got an Error page

I just updated my High Sierra through the Apple App store, and it downloaded, and was in the final moments of finishing up, and the screen suddenly went grey with the circle with a line through it. What happened? The computer will not start, and I was told to restart it but the only way to do that is to push the on off button and shut it off, then turn it back on. I have tried letting the battery go dead and then plugging it in, but it goes right back to the update, and then the grey screen.