This week's book giveaway is in the OCPJP forum. We're giving away four copies of OCA/OCP Java SE 7 Programmer I & II Study Guide and have Kathy Sierra & Bert Bates on-line! See this thread for details.
It could be a lot of things. The first thing to do is rule out your workspace. Can you launch Eclipse successfully pointing to a new workspace? If so, you can import the projects you need from your old one or pull them from version control. If not, reinstalling Eclipse might be needed.