Help. Error message 404 for my posts & pages

I am using the latest WordPress version (2.7). My blog was going great with no problems. Now whenever you click on any of the posts in the sidebar, or on the calendar, or any of my pages you will get an error 404 message. I suspect that I have some kind of disconnect from the database, but how would this happen, and what do I do to correct the problem? I am a newbie and don’t know a lot about that MySQL stuff.