Usually when you stop sharing a file it just turns red but stays in the library. If you choose to stop sharing a folder it will disappear from the Library but it's still on your harddrive. Try searching your computer for the file. Click Start>Search>All Files and Folders.

The default location of your shared folder is C:\Documents and Settings\'your account name"\Shared.

You can add folders to your LW library by going to Tools>Options>Sharing, and browse to the folder you want to share, click "select" then "apply".

If you accidently deleted the file from LW then it is gone and you would need a file recovery program to try and get it back. If that's the case, it'd probably be easier just to d/load it again

Edit: To answer the original question, to share a file you'd previously unshared, just right click the file and click Share File.

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