Mac Desktop: updated to latest OS and now in black screen with restart computer and then loading screen

I just updated my Mac Desktop to the latest OS and it shut down with a black screen that says the computer needs to restart press any key. I press a key and then it shows a loading bar that freezes and then it loops into the black screen that says it needs to restart press any key. It does it over and over. I tried to do the restart holding command R and it does the same thing. I tried holding down the power button until the apple logo appears and disappears but I cannot complete it because it goes into a loading screen that freezes. SO FRUSTRATED!