It sounds as if you got logged out for apparent inactivity while you were writing. The system can't tell if you're typing, only if you're requesting pages. If it hasn't "seen" you for an hour it assumes you went somewhere else and logs you out.

The easiest way around that is what pegasus suggested: check the "Remember me" box when you log in. You'll still get logged out for inactivity but you'll probably never notice. As soon as you click to submit or preview your PM, you'll be automatically logged back in again.

I've had browsers that were notorious for losing posts, emails, etc. with little or no provocation. The Firefox I'm using now, however, lets me recover "lost" posts about 90% of the time. A few times I've had the power in my neighborhood go out for an hour while I was typing, and still been able to get my post back afterwards.

If something that I'm writing is getting long and would be hard to reconstruct if it got lost, I may save it to a text file as I go just to be sure.

For me, perhaps the hardest part of writing something a second time is that I confuse the parts of it that I've actually put into the second version, with the parts that I'm still remembering from the first one.