mbp sleeping problem

I have the macbook pro 13" unibody OS 10.6.4

I usually don't turn it off, I close the lid and it goes to sleep.

Well last night I closed the lid, put it away, and I woke up this morning to see that it had completely shut off. So I turned it on again, used it for a few hours, and closed the lid and put it back once more for maybe 30-45 mins. I took it out again to find out that it had once again shut down. Both times it completely lost the wifi info and I had to reconnect manually and add the key again.

I usually keep it plugged in, so battery wouldn't be an issue.

Any ideas as to what's causing this problem?

 

no ideas? My macbook pro just shut off in the middle of using it this time....