I'm writing an update because I just solved my problem. My media never returned after following the advice I received from Tech Support. So, I tried the following and it worked.
I had invited a friend to be a contributor on my tree. Somehow, when she attached photos Ancestry.com through in a second period in her file name, yielding the following "photo..jpg" with 2 periods before the jpg extension. Naturally, my photos wouldn't sync with my linked tree because of the extra period. Lesson learned: check your media file and look especially at photos you added just before you could no longer view photos in your linked tree online.
Unlinked the tree with the double periods in some files, e.g., "photo..ppg"
Uploaded a new unlinked tree and all of my photos appeared once again online
Then, linked the tree and all of the photos appeared once again online
Perhaps this solution will work for others experiencing the same problem.