Can't boot to Mac OS volume after Bootcamp Windows install
Some strange thing happened.

I switched iMac off (like I did before for several times to restart) and let it be off for a time while I compiled fresh Mojave installation usb drive.

And after turning on iMac started to react on Option key, so I could choose MacOs volume to boot and it's booted successfully. Usb installation flash was plugged in.

Now this Option key boot menu is available without usb-flash plugged in...