1:
Right Click on the project -> Properties -> 
Project Facets -> Click on the Configuration Link 
-> Click on Apply Button -> Click on OK button.
The above steps should set your project as a Java project.
2:
I pressed Ctrl + Shift + H (Open Type in Hierarchy), 
typed in the class name, and opened it. 
The file opened in a new tab, and everything was navigable again.
Ref: https://stackoverflow.com/questions/14207074/eclipse-open-declaration-in-java-project