T520 Suddenly came back to life.

[thinkpadt520]

I took the battery out of my T520 and used only the AC adapter for a few hours. When i was finished using the laptop I shut it down and unplugged the AC then put it all away. The next day I needed to look something up on the internet, so I hooked it all up again and it would not power on. Anyway long story short I put the battery back in left it alone for a day or so then decided to see if it would power on and it did. would anyone know why it goes lifeless.