You've become disconnected from your JRE. I finally quit doing that (I hope!). I believe that you'll achieve happiness if you go to the project properties dialog/Java Build Path, select Libraries, click "Add Library" and re-add the JRE you want (assuming you have more than one).
The master list of JRE's is maintained by the Window/Preferences/Java/Installed JREs dialog, but I hope you didn't zap that.
An IDE is no substitute for an Intelligent Developer.