Shared data partition for Bootcamp no longer accessible
After a lot of trial and error and research I came across this post that solved my issue:
https://discussions.apple.com/message/19084645#19084645

Long story short, running fsck_exfat on the Data partition (disk0s5) indicated that the main boot region needed to be updated and it provided an option to update it.  Sure enough, this fixed it.

Prior to running fsck_exfat I tried the Verify Disk option on the partition in Disk Utility and it almost immediately failed saying "Invalid sector size: 0".  Clicking the Repair Disk option failed and said the disk needed to be reformatted.

After running fsck_exfat and updating the main boot region Disk Utility still showed the partition as "disk0s5" rather than "Data".  Clicking the Verify Disk option still gave there error saying the sector size was invalid, but the the partition name suddenly changed to "Data".  Then, clicking Verify Disk again said everything was fine.

Hopefully this will help someone else out there.  Or me, the next time I run into this...