FCP has a long standing bug where when you have the playhead in the Timeline over a clip and hit SHIFT + F for Reveal Master Clip, you'll inexplicably see this:
Reconnecting the clip will not help.
Trashing prefs hasn't helped me in this case.
Bizarrely, the clip will be fully visible in your Browser and/or Bin yet FCP will simply not be able to find it. It may be able to find other clips in the Timeline that reside in the same Bin as the errant clip, though.
I have no solution but it's an irritating, frustrating bug.
See, with the Timeline in the foreground and placing the playhead over a clip and tapping The F Key will perform a Matchframe where the frame -and Master Clip- that the playhead is over will appear in the Viewer.
But, hitting SHIFT + F with the Timeline in the foreground and placing the playhead over a clip and tapping F will perform a Reveal Master Clip; only sometimes it will just stop working. I have no idea why.
A workaround this to find a Master Clip and have it load into the Viewer is, like I mentioned, just place the playhead over the clip, make sure the Timeline is in the foreground (active) and tap the F key. You won't see the clip in the Bin or Browser but you'll be able to scrub around to see what else the clip holds footage-wise.
It's an incomplete workaround however especially if you need to see the following and preceding clips around the clip you're interested in. You'll likely find yourself doing this a lot while cutting a documentary and know that the footage is listed chronologically; a Reveal Master Clip would show you where the clip rests in accordance to the order the clips were shot in.
Another quirk is that if you do hit OK to "add a master clip to the browser" it'll do just that; and it'll find it if you load that into the Timeline (or Viewer, Reveal Master Clip works if the Viewer is the active window as well) and perform a Reveal Master Clip. Frustrating.
So there is a solution if you're willing to add a master clip to the browser (at root level) for every clip that suffers from this bug. But there's no guarantee that it won't randomly happen again. In fact, if you do add a master clip to the browser, place some of it in the Timeline, then for fun delete the newly created master clip from the Browser, FCP will toss up the Master Clip Not Present error.