MacBook Air turned off for no reason

Right, it might sound like the typical shut down for no apparent reason.

But in my case, I was listening to music whilst browsing the web. My Mac lagged for a second and then I tried to keep moving the mouse, but nothing would respond. After about 10 seconds my screen goes black. I wondered if the screen just went or something.

After about 5 seconds, I pressed the power button and it booted up. I logged in like normal, it loaded like normal and reopened and left everything the way it was.

The part I don't understand is why did it just freeze on me and why did it just shut down for no reason.

The battery is fine, it was replaced a few months ago. The charge was on about 31% and it came back on with 30%.

My Mac is a MacBook Air Mid 2011 256gb. I know it's an old Mac, but it's my first Mac I've owned for about 8 months.