macbook air goes sleep randomly

Hey guys!

My macbook air started to go to sleep mode randomly. It started on Thursday, it went to sleep, I couldn't wake, tried to press the power button for 10 seconds, nothing. After that I closed it, to remove the case on it and I just wanted to try again, then it turned on. From that time, it did the same some time, sometimes I just had to push the power button and it turned on again, sometimes I had to close the macbook, but after that everything came back. I reinstalled the system for the newest one. After that when I was watching a film online it went sleeping, but the movie was still going on, I closed it, and the voices from the movie was still on.

I have macOS High Sierra 10.13

Thank you for your help!