Did you have a return address on it, just in case you got the destination address wrong?
I assume so--it was mailed to me by someone else and she is now checking with La Poste to find out what, if anything, can be done to locate it. (Had I mailed it, I would have sent it with tracking capability.)
Thanks to all. dodo's post gives me hope that it's sitting on a desk in a US agency awaiting a blessing so it can get back in the mail stream.