My macbook air 11-inch mid 2011 turns off randomly.

Hi!

My MacBook air 11-inch (mid 2011) turns off randomly. It's not happening while i use it, but after I close my mac and open it again after some time.When I open it, it's turned off. Not sleeping. The mac does not respond to any key except the "turn on" button. When I press it, the mac turns on.

How can i fix this problem and what can i do?
(by the way, I checked the firmware SMC and it's up to date.)

thanks!