Well, I'm assuming that non-use of the iMac for a couple of hours, a black screen and no fan noise means that it has indeed completely powered down. Normally tapping the space bar brings it to life with no fuss.
I may be on the wrong track but I'm now wondering whether it has something to do with 1Password and its Safari helper app which crashed this morning. 1Password (the MAS version) has been a bit unstable and seemingly inconsistent lately and I'm about to run Agilebits' 1Password Troubleshooting app just downloaded from their website.
