Why is the green led blinking on my M3N72-D motherboard

The flickering green LED on the motherboard is illuminated by the standby power supply, a small completely independent, always on, 5 volt power supply labeled 5VSB usually, output is on the violet wire.

Here's some more detailed instructions about how to check a PSU.

Try to borrow a known good PSU of around 550 - 600 watts. That will power just about any system with a single GPU. If you cannot do that, use a DMM to measure the voltages. Measure between the colored wires and either chassis ground or the black wires. Yellow wires should be 12 volts. Red wires: +5 volts, orange wires: +3.3 volts, blue wire : -12 volts, violet wire: 5 volts always on. Tolerances are +/- 5% except for the -12 volts which is +/- 10%.

The gray wire is really important. It should go from 0 to +5 volts when you turn the PSU on with the case switch. CPU needs this signal to boot.