MY LAPTOP TURNS OFF BY ITSELF

I bought my laptop(asus x52j)back in 2010 or 11(I don't remember)but in 2012 a problem occured with the battery so I always have it plugged in since then.I never experienced this problem before:while I was watching a youtube video,my laptop turned off.For a second I thought it was a blackout but the lights were still on.I turned the laptop on but it was operating slower than normal,so I restarted it.Everything was running fine for about 10 minutes after the restart but then it turned itself off again.What should I do?Please help me!!!!!!!