I have multiple email accounts. On the recent wp10 upgrade (14393.189) which I think occurred on September 23, I could only see contents of 1 email account. Various combinations of that: Sometimes all email accounts were shown in the list, sometimes not. When I switched to another email account, it only displayed the title and then the contents showed my Hotmail. Sometimes it just needed to sync. Other times it crashed.
I knew the emails and folders for the other accounts were still there as I got some notifications on the notification screen that I had emails only gmail. When I clicked that, I was able to see the gmail but nothing else. (So that is work around: Send yourself an email and see the notification – but did not seem to work for exchange/outlook based emails).
I tried hard reboot but that did not work.
Fix was to delete the account that was showing (my Hotmail account, luckily with no contacts attached to it) and then reinstall it (had to do that twice, as the hello/pin authentication ended up with a blank white screen. Inbetween the 2 times, I also used the edge browser to log on.