To rebuild the Outlook search index:
Open Outlook and go to File > Options > Search.
Click Indexing Options > Advanced.
Under Troubleshooting, select Rebuild.
Wait for the process to complete (it may take time based on your mailbox size).
Restart Outlook and test the search function. This process forces Windows to re-index Outlook files, often resolving search issues.
To know more about outlook search not working and how to fix it read the blog.