High Sierra update frozen will not start up

Just updated my iMac with high Sierra but it froze before the end now it will not start just goEs back to the same page but bar and updating totally stuck. I have USB backup but the iMac will not go into the startup page to choose restore. Tried a combination of keys I have

Ctrl, Fn, the one with the cross symble and Alt on my usb keyboard and iMac old enough to run Sierra so what ever year that was to be honest I am lost without my iMac..

Any advice welcome I can't afford to buy another and my iPad mini was on ever a stand in so links bookmarks all out of date.

HELP

Peter