The latest software update has caused my iMac to be unable to restart.

Can anyone advise me how to restart my 7 year old iMac, please.

Whilst trying to download the latest software update my machine needed to be restarted. However, it has been unable to do so. It is stuck in a loop where it tries to start, then aborts. It displays the message, "Your computer restarted because of a problem. Press a key or wait a few seconds to continue starting up." When a key is pressed it starts restarting before reaching the same message again ad infinitum. Please help