I left my PC on sleep over night and woke this morning to the power light on my PC tower flashing on and off and the EZ debug CPU light lit up on my motherboard. I turned the PC off and tried to turn it on and the light on the motherboard was still lit up and there was still no output to the monitor and there was no POST. I've had this build for 4 years and it's been working smoothly up until today.
Check the motherboard manual to interpret what the EZ debug LED and flashing power light may mean.
Try another known working cable to monitor.
Try another monitor.
Try connecting motherboard video port to monitor.
Try booting into safe mode.
I should probably have made it clear in the main post that there's no POST. The manual doesn't really give any troubleshooting advice about the debug light and every occurence of it that I've read online is from people with new builds who haven't assembled things properly which wouldn't be my situation as it has been working fine for the past 3-4 years.