Running High Sierra 10.13.6, I tried to install Security Update 2019-003, but it would not install because the computer gets stuck (blank screen, responsive pointer) when trying to restart. Now I see that the iMac will not shut down or restart under any circumstances. I tried internet recovery re-install of High Sierra, but it also got stuck during re-install and did not complete (probably due to re-start problem). Tried again in Safe Mode, no success. Tried resetting SMC and NVRAM. Apple diagnostic - no issues found. Disk First Aid - no problems.
Apple support suggested trying to install Mojave. I prefer not to at this time, but even if I did, I'm afraid that the restart problem will prevent Mojave install to complete.
Any suggestions on why restart and shutdown aren't working and how to fix?