I am using Outlook 365 and have been for the past few years - this is on a Windows 10 Professional system. Everything has worked fine up until about a week ago and I can no longer use the search function - every time I put something in, it comes back no results. I have re indexed the information as well as going into the Options, Search - unchecking Outlook and re-checking it, which seemed to work for about that long but went back to no results. If I totally uncheck the box, it will give me a message about Something went wrong and your search couldnt be competed - however it searches thru every folder bringing up stuff
Now open Outlook, Options, Search and make sure the settings are correct. Current Folder may not find much, but Current Mailbox or All Mailboxes should find what you want.
This is how I set Outlook.