iMac screen shuts off after 10/15mins of working

Hi, I recently changed from hdd to ssd . After 2days my screen keeps going off after 10mins of boot up. I can see my keyboard/mouse working, even music/sound keeps continuing playing/copied files. And after another 10mins the mac shuts off.

I can restart it , but again same thing 15 mins to 30mins and screen goes off .

What might be the problem? How do I identify it?

Did reinstall, reset everything , double checked cables. All four led light (green)working on the board.