Replying to Macbook problem.. constant sleep.

So two days ago I closed the lid to my first gen Macbook (came out two years ago) and went for a walk.  Came back an hour later and opened the lid and it wouldn't turn on.  The sleep light was still on even when the lid was up.  So now whenever I plug it in and try to turn it on it does the usual start up sounds with the flash of the screen and then it just goes into sleep mode.. Anyone ever had this happen or know anything about it?

My battery died about six months ago so I've been running off the wall since then.. don't know if that would have anything to do with it.

Any help appreciated!