Your trees have not vanished, they are safe and sound on your hard drive.
All that appears to have happened is that the list of trees link has broken between FTM and your folder that holds your files.
All you have to do, like any other program, is do a File>Open in FTM and navigate to where you have your trees stored on your hard drive.
If you don't know where they are, then use the Windows Search and search for *.FTM. That will produce a list of them.
Using File>Open will also relink them to the recent tree list in FTM
Really, it is not rocket science, just basic Windows file management.