iMac stuck on load screen, can't reboot in Recovery Mode
My iMac was working fine until I turned it on after a weekend. It is stuck at the very tail end of the load screen. I can't reboot in Recovery Mode because it isn't getting to the point where the Mac "on" sound engages. I've turned it off and off and unplugged it.
What can I do?
iMac 21.5", macOS 10.13