Errors in Eclipse platform (Selenium/TestNG/JAVA buildpath)

I was doing my tutorials on Selenium JAVA in Eclipse platform. After downloading new software TestNG, ran into issues, where 2 errors are showing up. Description Resource Path Location Type Invalid classpath container: 'TestNG' in project 'seleniumwebdriv' seleniumwebdriv Build path Build Path Problem The project cannot be built until build path errors are resolved seleniumwebdriv Unknown Java Problem Description Resource Path Location Type

Apart from the errors, The project folder isn't showing up the previous files. How can I fix these errors and start working with TestNG.

The window shows upon trying to open and close folder problems occured during save Cannot invoke "object.hashcode()" because key is null

Thanks.