iMac startup sound or display
I have a mid 2011 21.5inch iMac (Model A1311). I was upgrading from Mac OS 10.12 to 10.13. During the install I was in another room and heard a long tone. When I looked at the iMac the screen was blank. Pressing a key on the keyboard didn't activate the display. I left it for about an hour then I forced it to shut down by holding the power button.
On pressing the power button the computer powers up - I can hear a faint whir of the hard drive or fan. But there is no startup sound and the display remains blank. I can switch it off by holding the power button for a few seconds.
I've tried resetting the SMC but holding down command option shift and the power button. Still blank screen and no startup sound.
I've tried disconnecting the mains cable for 15 seconds.
I've tried disconnecting the keyboard. It's the only thing connected.
I've tried removing the ram to see if I get the ram missing tone - I didn't.
Any ideas?
iMac, macOS High Sierra (10.13.6), null