“Not Found” – Help!

Hey all
I think I screwed up big time.
I was changing some stuff to make the page show the last update and all, and so I worked with the header.php file (I’m using the Dark City 1.0 Theme). Then I tried to figure out how to make certain pages visible only to logged in users. I added a “private” page to test it and deleted it. That’s when it happened.
I don’t know if it’s because of the private page, or a plugin, or my screwing up the code (?), but it won’t show me any posts or pages anymore – it does show the titles, categories, links, dates, subtitles etc, but upon clicking on it it just says “Page/Post can’t be found”, even though they’re all there!
What can I do? I already tried to copy my backup header.php back to the actual one, but it won’t help! 🙁