Is this a common problem? I saw on Webmaster Tools that "Googlebot can't access the page".... Well, than I checked the robots.txt, but I couldn't find it anymore. Why is it gone? And how do I add it back?
Yes, put it in the root. Make sure you customise it as Chris suggested to suit your needs. If you're unsure about this, search these forums as there are several threads about robots.txt files.