Same thing happened to me. They switched without my permission. Woke up and found the emails from the last 4 years were missing! I reverted to the "legacy" Outlook. If you haven't solved it yet, simply go up to the menu bar, click on "Outlook" for the drop-down menu. If you're still in the "new outlook," it will be checked. Click that to uncheck, and your legacy outlook will return with everything intact.
When this first happened, I actually got a hold of a tech person in Microsoft, and he was very honest! He said they're still working out the kinks in the New Outlook, so he advised me to revert to the legacy.