Click Rescan in the Projects tab of the Properties. This should recover all missing and deleted projects.
If there are still missing projects, restore all CSP files from your latest repository backup as follows: Put all CSP files stored in the backup repository top-folder into the Recycled\Removed folder of your current repository. Then, click Rescan in the Projects tab of the Properties.
Alternative Method:
Project definitions are stored in the STP files in the project repository top-folder. You may simply re-define the project in the Project Library and confirm you wish to connect to an existing project.