Projects are not found because of their location in a special folder (i.e. workspace folder). They are shown when they are referenced in the workspace metadata. So you have to import the existing project into the workspace.
Much better: Check-in your projects into a version control system then you can check out easily if something happens. With SVN it is as simple as creating a folder and "declare" it as repository, you don't even need to run a server.
Joined: Jan 25, 2010
Which application allows for "creating a folder and "declare" it as repository"?
I suspect it's Tortoise.
Hauke Ingmar Schmidt
Joined: Nov 18, 2008
It's an svn feature to access repositories by file protocol without a server, i.e. file:///path/to/myrepo. (with myrepo the folder for the repository to be created) does the job. You can access this by Tortoise, too. If I remember correctly you access it by a folder's context menu -> Tortoise -> Create Repository here.
For me this easy serverless access is very helpful. I even have some USB sticks with different repos instead of "just files".
Oh, but not to be mistaken: This does not solve your existing problem, you just have to import the existing project for now. It's a suggestion for the future.