Emails in inbox were deleted and 1500 emails now appear unread that were previously read and deleted.

I was viewing my Comcast email account in Apple’s Mail app. While I was viewing it, the email I was viewing was suddenly deleted as well as all of my emails, both read and unread from the last 30 days. My xfinity mailbox also has these emails missing. 

As this happened, my unread emails went from less than 200 unread emails to 1500 unread emails. Many of which I had previously opened and deleted. Going back years. 

What can I do to recover the emails that I am missing? Only some of them are in my deleted folder. Why did my emails suddenly appear as unread months and years after I read them and deleted them?