The proper procedure to isolate the problem. iMac does not start

The computer has experienced a terrible fall, and the display is broken. So I removed the display and HDD and tried to plug in the power cord. The first test led on the motherboard is ON. Then I tried to switch it on but with no luck. No other test leds blinking, no fan.

I wonder what is the better way to resolve the further problem isolation.

Should it be able to start without builtin display when the external display is using?

UPDATE

So I did remove every secondary elements from iMac, the only motherboard and PSU left, checked the power button for continuity, but still without any success. iMac does not response to pressing the power button. Adding HDD, one RAM memory stick and external monitor via Thunderbolt didn’t change the picture. As far as I found out the meaning of these diagnostic leds is the following:

• AC input voltage (trickle power present) power supply
• DC output voltage
• GPU is functioning
• display panel is connected

So the first led can only tell that the AC power is presented. I hope I can do more the find the problem.