Mackbook won’t wake-up

OK I need some help with my Macbook.

I was online (WiFI) and I walked away with Safari still running. I came back an hour or so later and the screen looks like it’s asleep (its really dark but you can still see the web site that I was one last) and I can hear the HD making sounds.

So I tried to wake it up, force quite but nothing changes.

Any ideas?

try turning up the brightness, if it went to sleep you wouldnt be able to see the websites in the background. Ive had this happen where the comp wouldnt wake up, but somehow the brightness was all the way down.

I held down the power button for awhile and it fixed it.

I still am wonder what caused the problem.