Mail messages missing...

Don't know if this is a Sierra issue or not, but this is the first time it has happened, and within 2 days of upgrading to Sierra.

My main mail IMAP mail account was working fine this morning. About 49,000 messages. I glanced over at the Mail window this afternoon and it was completely blank. No messages. Title bar says "0 filtered messages".

The messages appear on my phone/ipad/webmail, but not in Apple Mail for this particular account. Other accounts are displaying fine.

I have tried rebuilding the mailbox, no change.

I have deleted the IMAP account and re-created it, it re-downloads the 49,000 messages, but doesn't display them.

The messages are still on the IMAP server, I can do an "about this account" and it shows there are 49,000 messages in the inbox.

So, now what?