computer poweers on, no post,

[Gray Sarge]

Powers on fans, no post, blue screen, one long beep, power supply checks good , memory and video card reseated, F2 and F12 no effect, all cables connected.

 

[johnbl]

the power supply sends a signal from the power supply to the motherboard that indicates the power is ok. if the signal is not set then the motherboard logic will keep the CPU reset and will not let the CPU start.

This means the fans can start but not the CPU. You want to check all the power connections from PSU to the Motherboard and make sure they are all connected. Sometimes one line of the power supply will not be connected or will have failed.
I have also seen cases where there was a short circuit on the motherboard and the motherboards circuit protection logic kicked in to save the motherboard.

Basically, make sure everything is plugged in, including any required supplemental power connections to the GPU and CPU. And that the CPU fan and PSU fan is working as expected.

Also, I have see people problems when the PSU is plugged into a power strip rather than directly to the wall.
(when the power strip was faulty)

Powers on fans, no post, blue screen, one long beep, power supply checks good , memory and video card reseated, F2 and F12 no effect, all cables connected.