MacBook sleeping with not attached to external monitor

I have a 2.16 GHz MacBook Pro running 10.5.1 with 2GB of memory.

When it is connected to an external monitor it works fine. However, as soon as I disconnect it, it goes to sleep in about 10 seconds.

Any help would be appreciated.

I realized that it thought the lid was closed when clearly it was not. I installed Insomnia, and that seems to be working. Thanks to all who viewed this message.