I also had this problem once. Arises when you check-out a project from a source control repository and some of the configurations held by the project creator are not shared among team members. By chance, are you using HibernateJPA as the JPA implementation? In that case, check the following:
Go to your project Properties > Hibernate Settings
, once there, check the combobox below the "Default Hibernate Console Configuration" label and select a console configuration. If none console configuration exists, go to Run > Run Configurations ...
and under Hibernate Console Configuration
create a new Configuration, specify the project name, in the "Database connection:" combobox select "[JPA Project Configured Connection]", click Apply and return to the "Hibernate Settings" settings page of your project configuration.
Once everything is done, clean your project (maybe it has be done a couple of times before the error dissapears though, due to some eclipse issues), if the project doesn't refresh properly, restart Eclipse (yes, it's kind of extreme but sometimes it just works).
If you're using Maven and the m2e plugin, right-click in your project and goto to Maven > Update Project Configuration...
and wait for the project to been refreshed.