1. Windows 7/8/10 --> control panel --> Indexing Options --> Advanced --> TroubleShoogint (Delete and rebuild index) --> REBUILD
2. If you will not find emails you should:
- Stop windows Search service.
- restart Outlook
- Execute cmd with Administrative rights
net stop wsearch - Set to 0 this key:
HKLM\Software\Microsoft\Windows Search\SetupCompletedSuccessfully
3. About Outlook search returns No matches found you can review following KB 2769551 Microsoft article with all step by step procedures:
https://support.microsoft.com/en-us/kb/2769651
[Update 2020.04.09]
Outlook for Office 365 Outlook 2019 Outlook 2016 Outlook 2013 Outlook 2010 Outlook 2007Fix search issues by rebuildling your Instant Search catalog
Outlook for Office 365 Outlook 2019 Outlook 2016 Outlook 2013 Outlook 2010 Outlook 2007https://support.office.com/en-us/article/fix-search-issues-by-rebuildling-your-instant-search-catalog-213a2728-0ef4-427a-9bb2-aed329a59b17
Troubleshooting Outlook search issues
Outlook for Office 365 Outlook 2019 Outlook 2016 Outlook 2013https://support.office.com/en-us/article/troubleshooting-outlook-search-issues-2556b11f-f4d8-46be-b0a7-de33a3f4f066