This message has the following cause and solution:
Click OK if you want to try to find the project. If the project has been deleted, choose Cancel. Once you click Cancel, the database will be detached from source code control, and you can re-add the project to source control (but you will lose any historical information on this project).
If you can find out who deleted or moved your project, you may be able to recover the project and its history (for example, if your source control administrator has archived your project to a backup tape).