iMac unable to restart after trying all solution...pls help me.

Hi All,

Pls kindly help me as i’ve ran out of solution as to what i need to do with my imac.

Problem: My imac will go into shutdown mode when i restart my iMac. I am not able to boot up my imac despite pressing on the power button for more than 20s. The only way it would boot up is when i pull out the power cord at the back for more than 30s and wait 5s to turn on the power supply. When it boots, it takes some time before it comes to the login screen. I’ve done a PRAM reset using option + Command + P + R and done disk utility repair and reload my Big Sur OS. The fan goes on when it boots. I have also more than 1GB disk space left and more than 10gb ram unused on my memory.

I sent it to the 3rd party repair shop who specialise in Apple products. They have done quite a few tests and they said there is nothing wrong with my power supply as the LED lights on the power supply board are on and motherboard seems fine too. There is just no power going to the harddisk.

What else can i do? Is it PRAM battery issue? Hard disk need replacement? or is it OS or software issue?

Hope you all who experience this can shed some light to my problem. Thank u!