@rajeshbonde
As is evidenced by the lack of response to your post in all this time, there is no simple fix for this that you can do yourself.
If the PC is several years old and you have been pressing the button to turn it on every day, most likely, the contacts inside the monitor have worn to the degree that now, you can't short them with the button reliably.
Since there are no diagnostics for this, the only way to fix it is to have the switch replaced -- but that will cost you and we are unable to provide any estimate of that cost.
I am a volunteer and I do not work for, nor represent, HP