Single-user and multi-users OSs:
In the latest Windows code, set the following location in the registry to 0
HKEY_CURRENT_USER\SOFTWARE\Microsoft\Windows Search\SetupCompletedSuccessfully...
Updated May 11, 2022
Version 1.0
Blog Post
If you are roaming your profile (and even if you are not) you can reset the user's Windows OS search index [which indexes the files on the system as well as Outlook messages and possibly other things of which I am not aware]. So basically, if a user's search index seems to be corrupted then you can do this and let it rebuild and if it works then it was corruption. If it doesn't, then there are other issues to be investigated and a ticket should be opened. This is a fairly easy and non-destructive method. It doesn't delete any of the users work files, just the index. And if the index wasn't working already, then there is nothing at all to lose.
This only applies if the per-user index is being used which is available (by default) on Server 2019 and above as well as Win 10 and 11 multi-user OSs. You can also use the per-user index on Win 10 and 11 as per this post (https://techcommunity.microsoft.com/t5/fslogix-blog/fslogix-single-user-os-search-roaming/ba-p/3346787).