I’ve rendered this file and the result was like this, with the grease pencil strokes.
Then I rendered it again and the strokes just didnt appeared anymore, I checked all the grease pencil layers and they were all there, but with no strokes in any of them. All strokes were deleted, but not the layers they were in 
I’m sorry to hear that, that’s so frustrating!
Not sure how familiar you are with grease pencil but is it possible you moved forward in the timeline and new blank keyframes were created?
Omg!!! Thank you very much!! This was exactly the problem, I just pressed the spacebar and my frames changed, so i had to search a little and find then!! thank youu 
Nice, glad to hear it! 
