Just updated on my iMac and now have continual 'You need to restart your computer ...' What can I do?

Just done routine update on my iMac, it asked me to restart and now have continual 'You need to restart your computer ...' What can I do?

on a grey screen I see:

panic (cpu 0 caller 0xffffff800064ba7b): "Unable to find driver for this platform: \"ACPI\".\n"@/SourceCache/xnu/xnu-1699.26.8/iokit/Kernel/I0PlatformExpert.cpp:1 504

Debugger called: <panic>

...

can't perform kext scan: no kext summary

BSD process name corresponding to current thread: Unknown

Mac OS version:

Not yet set

...