I also have had this EXACT problem. Works fine for about a month. I populate the blog, everything is good and one day I get the welcome screen.
Looking at the database shows that the wp_options and the wp_posts are "in use". I can't access either of them.
I've had the exact thing happen on 4 blogs now. Any ideas at all? The only thing that my tech people can come up with is that I had set the content file to 777 instead of 775. They aren't seeing any activity other than mine. Do we think the entire problem is me getting hacked due to the bad permissions?
I've had the databases so wrecked, I had to start over. Only once we could fix it.