Macbook Air Shuts down after I close the Lid

In normal conditions, if I would shut the Lid off of my macbook, it goes to sleep, and when I open it up, the login prompt is shown. It was OK.

But now when I close the lid of macbook air, it sometime opens right, but sometimes it shut downs on itself losing unsaved files and open apps. and for sometimes it takes too long to start up.

What might be possible reasons and remedies?