This appears to have happened to me too
What is interesting is that for me in the Control Panel, under Disk Usage / Disk Usage By Address, I have email accounts listed twice -- once with "no address (name)" and correct disk/file usage, and once with "email@address" with 0 file usage.
My guess is that webmail/MAPI is connecting to the latter, resulting in an "empty" mailbox. Hopefully the first indicates the data is still there somewhere...
Not sure if it is related or not but I received an email on November 5 that my account was being moved to a new server. I wonder if there was an issue moving the corresponding mailboxes.
Opened a support ticket 4 hours ago -- no response yet.