if you do a search you will see this question asked many, many, times.
with no real answer as to the cause.
or a permanent solution.
you will find several suggestions to try with the most common being the ON 30", Vol+ ON > 15" routine which I am sure you have tried.
this is my experience:
- I had the issue about a year ago and the Vol+ ON worked for me and then I forgot about the issue cause it never returned until...
- after a recent update the problem came back. wouldn't wake, and twice turned itself off while in tablet mode. nothing worked to wake it.
- one day I played with it for hours trying different combinations and it finally woke!
- I went into power settings and told itNEVER to sleep.
it has been fine for about a week now.
I turn it off several times a day and it starts again ok each time. I just don't let it sleep.
There has also been another MS update. coincidence?
this is my current thinking:
- it isn't broken
- it isn't the battery or a hardware error
- it is a software error
- it might be due to MS updates
- a clean install might fix it, stop it from recurring, haven't tried it myself
would be great if MS could solve this.