Unable to "wake up" HTC One M7 screen but the phone is/was ON

Hi All,
This will be my first post in this forum.

This morning i was using the phone after it was 100% charged. It was good until when i wanted to make a phone call, tried to switch on by pressing the power button, then i realized the screen didn't turn ON. To check if phone is ON, i made a phone call from my office desk to my HTC one and i heard ringing sound over the desk phone. But my mobile HTC one did not wake up. I also didnt hear any sound in my HTC one. Later on i believe the phone switched off and now any trial calls go direct to HTC's voice mail.

Steps tried:
1. Tried to hold the power button for long, doesn't work.
2. Tried the Volume Down + Power button long hold to see if it comes up. But it didn't.
3. Also, tried Volume Up + Power button long hold, which also doesn't work.

I read in the same forum that I need to let go off the power button after 2 seconds after the long press, when the phone is switched OFF. I need expert suggestion.

Thanks!!