Windows 7 Pro, 8GB ram, SSD HD, upgraded from Office 2007 to 2016 & now the search function isn't working.
Ran office updates & when trying had one time where I had an option to search on server & an item was found, but for the most part, I'm getting 'no results'.
Running Exchange 2013, if that matters.
Have you already tried checking if Outlook items are indexed ?
If the index is still busy building you won't see search results immediately , here's an external article on troubleshooting the issue you're encountering:
How To Fix Outlook 2016 Search Problems