nothing on the screen except a message to insert the boot disc.
If there was nothing on the screen there would be no message and the screen would still appear to be off.
The message about inserting the boot disk doesn't necessarily mean there's a fault, but there's a bigger issue: you said you don't have a CPU fan. I hope you're not running the bare CPU with no heatsink at all. That's a very good way to make things go very badly, very quickly.
CPUs thermal throttle till they shut off, its been that way since P3 days, Athlon didnt receive this till it was implemented into their mobos to shut down immediately.
First things first, read your motherboard manual to gain an understanding of what all LEDs/display error codes represent, their locations.
Take all necessary parts required to start the motherboard up outside of the case -Power supply, motherboard, cpu, ram, graphics card/igp, keyboard, mouse, monitor. Put motherboard and other parts on a non conductive/non metallic/foil bench/table, elevate motherboard on non metallic/foil box.
Remove processor heatsink, inspect the lga socket for any bent pins, blow out any dust gently using a can duster, DO NOT TOUCH THE LGA SOCKET PINS. Reinsert the cpu correctly with pin 1 aligned, clean heatsink and cpu off. Apply fresh thermal compound to cpu and reattach heatsink. Pull the ram, check pins on ram for tarnish/gummy/dust, clean gently if any found using electronic contact cleaner or at least 70% isopropyl alcohol, blow dust off.
Check ramslots on mobo for cleaness- dust, blow it out, same with pci/pci express slots where gpu goes. Check gpu pcie contact edge, make sure to be gentle and be mindful of the shiny/dull rectangular capacitors-DO NOT KNOCK THEM OFF. Clean using contact cleaner/isopropyl alcohol.
Locate the clear cmos/bios reset jumper (read your motherboard manual) place jumper on right position for bios reset. Pull CMOS battery for 30 minutes, if you have a known good CR2032 battery, put it in.
Hook everything necessary up to get machine to post correctly. Once you do, shut down and hook up your Storage drives.