If you somehow happened to delete the .idea folder, and you could not see anything about the modules, one way to solve the problem is:

1. Go to File->Project Structure;
2. Select Project, modifiy SDK,Languge Level etc;
3. Select Modules, press + button to import the (parent) module.

In step3, if your project has a parent-child module relationship, importing the parent module is sufficient. Plus,I have tried File->New Project From Existing Sources->Import project from exteral model,but it did not work for me.