create a new folder that has no subfolders and save your fla into that. retest.
if that fails your fla is corrupted or your flash is corrupted. (try another fla to see if it works ok.)
if you have a working swf, you can decompile it. (google sothink's decompiler.)
you can try renaming your fla to a rar file and using winrar to repair it. that seldom works but it's your only chance at a 100% satisfactory solution.
and you can unrar you renamed fla and may be able recover some of its assets.
and, in the future, don't every let this happen again with flash pro or any other software you use to create time-consuming products. save periodically with an appended version number that you update with every save so you never end up losing much work.
Sorry, I tried your first method and failed. My flash sits in a folder without any sub folders to begin with. Feels like every time I exit the Flash, all my images are gone from the library, no matter they are in the same folder or not, one image or multiple.
Thanks for your offering to help!
WinRAR could not fix it either.
there's not necessarily a satisfactory fix (but you should still follow the suggestions).