1) Open a project.
2) Goto Files Tab and highlight project folder.
3) Open Properties Window if not open.
4) Rename Project Folder in Properties:Name field. This renames folder but does not rename the project name. The project name always shows for project folder in the Files Tab.
5) Return to projects tab and hover mouse cursor over name of project and see if folder name reflects new name. Sometimes it does, sometimes it doesn't.
6) Regardless of step 5, right click project and choose Run.
7) Result: Error: Could not find or load main class com.sun.example.mvc.Main
Java Result: 1
8) Look at project Propertis>>Run>>Main Class>>Browse Button. No classes are listed.*
9) By now if you hover the mouse over the project name in Projects Tab, it may reflect the new folder name, but it still won't run even if the name is correct.

Solution 1: If you close the project and reopen it, it may run OK and everything is well. This may or many not work, if not try Solution 2.

Solution 2: Close project and close NetBeans, restart and reopen project. This may or may not work, if not, try Solution 3.

Solution 3: Close NetBeans and delete .netbeans/dev/var/cache/ directory. Restart and project should run.

*One time when project would not run, the classes did show in Properties>>Run>>Main Class>>Browse Button, but selecting the Main class again did not work without closing project and reopening it.