I use my quick search a little too much, but for some reason in the last few days the drop box on my quick search has gone.

It is the one that allows you to quickly refine your search for "From:" or "Subject:" and "Keyword:".  I can add the criteria to my search manually but curious if anyone else has seen this.

I am using windows 8.1 and it is an exchange mailbox.

Hi,

Basis on your description, do you means search folder with From and Subject parameter? Figure as below:
 
I want to double confirm whether all Outlook client or some special user experience this question.
Please try to open search tab--->select properties what you need in More Properties, figure as below:
 
More details about Find a message with Instant Search, for your reference:
https://support.office.com/en-us/article/Find-a-message-with-Instant-Search-56c08e23-8035-4ce9-99f3-ca444848a99c?CTT=1&CorrelationId=778e0ddf-19ce-4cde-a163-24873c31d11c&ui=en-US&rs=en-US&ad=US

Thanks

No, what I mean is when you click on the quick search bar it offers three quick options to refine your search.



These are what have gone.

Hi,

Sorry for my misunderstanding.

For your question, it may be related to Windows Index. Please try to open Control Panel\All Control Panel Items\Indexing Options, click Advanced to delete and rebuild index.
Besides, we can run Outlook with safe mode to prevent virus and add-ins effect, or recreate a Outlook profile for testing.

Thanks