If that is the monastery in Ethiopia, the legend is that is there, but no one has seen it.
The part I found intriguing is that in no part of the Bible does it even mention that it was lost, taken or stolen.
It doesn't have to be lost , taken or stolen.
http://www.biblegateway.com/passage/?search=1 Samuel 5
There is a chapter about it being stolen.
The takers all got diseased and aflicted.
And their Idol kept falling over.
So the enemy placed the Ark on an oxcart hitched to two hefers who had just calved.
The cows walked away from their calves , and headed for Isreal.
When it arrived there was great rejoycing.
They sacrificed the hefers on the spot on a pyre built of the oxcart.
Then solomly promised to never be careless with it again.
I kinda felt sorry for the four cows.
http://www.biblegateway.com/passage/?search=2 Chronicles+12&version=NIV
By the time of King Rehoboam it must have been gone without being stolen .
The theroy I like best is the idea that a faction left with it and the scriptures about it after that were written by that faction.
Ethiopia has had Christians since way back when and Jews from before that.