it's not that out of the ordinary.
you caould have accidentaly delted the bin
it's a bit odd that you dodnt get a warning when the bin was accidnetaly deleted:
"the clips you are abput to delete are master clips"... etc, etc
but you dont say what version of FCP you;re using.
you wouldnt get that waring in FCP3 or less
maybe the bin has been accidentaly put *inside* another bin?
you cou;d do a search in hte browser for that.
OR
if the clips in hte timeline are still "affiliates" of the "master" sync clips which are hidden somewhere in hte project,
park on a clip in the timeline,
hit Shift F.
the master clip will be revealed and you would have found the sync clips bin
------------------------------------------
remember that EVERYTHING in FCP is just a pointer back to your media files on their hard disks.
you may have lost the bin the sync clips were in, but the media is stil online,
and the squnces still have their edits in them.
if you wanted to re-visit a scene, and needed to look at the sync clips again, you'd be at a diadvantage.
do you have an older version of the project that you cold open and look for the sync clips bin?
you could check your autosave vault.
cheers,
nick