How to open an existing project in Eclipse?

From the main menu bar, select command link File > Import.... The Import wizard opens.
Select General > Existing Project into Workspace and click Next.
Choose either Select root directory or Select archive file and click the associated Browse to locate the directory or file containing the projects.
Under Projects select the project or projects which you would like to import.
Click Finish to start the import.

Maybe you have closed the project and configured the project explorer view to filter closed projects.

In that case, have a look at Filters in the Project Explorer view. Make sure that closed projects are disabled in the "Filters" view.

from Eclipse main gui: select "Window->Show View->Other->General->Project Explorer" Double-clicking on "Project Explorer" brings up the "Project Explorer" window which shows every project in your workspace. That worked for me.

Good luck.

In case you closed multiple projects and trying to re-open all of them then in Windows->Show view-> Navigator