This is a signal from the monitor only and will persist until it senses a connection with the video display. What's missing in your question is whether or not the PC seems to try to boot or does it not come on at all either. Such could be a BIOS setting as well. What needs to be known is if the control signal to the MB has been removed for some reason. There should be a BIOS setting allowing a restart upon power failure. Try setting to this if you have not done so.
I have a fairly new computer with a No Signal error at boot up.
I know that the monitor, connections, motherboard, video card, RAM, CPU, and PSU could all cause this error.
However, if I unplug the computer's power cord from the wall outlet for a second or two, plug it back and turn the computer on , the error disappears and the computer works fine until next reboot.
Could this point to a malicious PSU?
Any word or two shall be appreciated.