Lightroom :: Original File Missing After Syncing Folder With PS Edit Versions?

I just noticed that some of my photos are missing from the library in lightroom, specifically the original versions of photos that have virtual copies edited in photoshop in folders that have since been synchronized.  Weird, and very anoying.  The files are still there (in wondows explorer) but when I try to re-import them lightroom doesn't see them, only the PS edited copies (.tif files).  I told lightroom to allow duplicates and it sees all the photos in the folder except those missing ones.  This does not happen to my PS edited virtual copies in folders that have not been synchronized.  It is not a matter of stacking, or at least Lightroom does not think the PS copies are stacked at all so I can't unstack them. How to get them back into lightroom (preferably with the accompanying development settings) and also how to avoid this problem in the future. 

PS: I import files from my camcorder using a different program because lightroom does not acknowledge that particular video file format.  That's why I have to synchronize some folders and not others.