Unable to boot

I have a 2015 iMac 5k which I believe is/was running Mojave. Last November I used bootcamp to install Windows 10 on a (I think) 200gb partition of the (I think) 1tb hard drive. All has been fine with no issues at all. It's been running in MacOS for the last few weeks without being powered down, just put in to sleep mode when not in use, which is how it's always been used.

Last night I watched a few hours of Netflix, put it in to sleep and went to bed for the night as normal. I've now awoken to the iMac appearing to be booting up, black screen with the white Apple logo and progress bar below stuck around 1mm filled. I left it for half and hour with no progress so turned off from the button. Turned it back on after a few minutes and got a black screen with text saying "your computer restarted because of a problem. Press a key or wait a few seconds to continue starting up." In various languages.

It went to the apple loading screen, loaded about 1/4 of the way and restarted again. It continued in that cycle, sometimes just getting stuck on the loading screen forever, sometimes restarting itself.

I phoned Apple support who walked me through trying to start in recovery mode which didn't work, ctrl + R just elicited no change in response from the computer. Then internet recovery mode did look promising at first, it loaded to my network and started the progress bar with the spinning globe above. It finished the progress bar but where it should have then gone to utilities it instead went to the apple loading screen. After a second or two a few lines of very tiny text appeared in the top left corner but I couldn't get close enough to read it before the computed restarted and went back in to the error loop.

Apple have booked me a genius bar appointment next week, but I am lost without my computer so I'm keen to try and figure it out before then!

I've tried the internet recovery a few more times to try and recreate the text in the top left and find out what it said, but it's only shown once more but disappeared even quicker and when it restarted it showed a no entry sign before the error message and another restart. I've also had a few times that the computer has restarted but no chime has sounded. After a few restarts the chime reappears.

I've tried booting in Windows to no avail. It tries to boot in recovery mode automatically without me pressing f11 but within a few seconds restarts and reattemps to boot in macOS getting in to the error/restart loop.

I'm desperate! Anybody have any ideas I could try?