Why Isn't Spotlight Searching My Files?

Thank you, Barney-15E. I did try re-indexing How to rebuild the Spotlight index on your Mac - Apple Support

but received a "Privacy List error (null) The item couldn't be added or moved because of an unknown error."

I rebooted and received the same error, so I tried re-indexing through the command line but can only manage to get "Index is already changing state. Please try again in a moment." and "Operation not permitted while System Integrity Protection is engaged." messages.

I tried that one but received the Error: "Index is already changing state. Please try again in a moment." I waited until today then tried it again but got the same error message.

But I think I might have found a solution:

sudo mdutil -a -i off

(Turns off spotlight)

sudo rm -rfv /.Spotlight-V100

(Removes the index files)

sudo mdutil -a -i on

(Turns Spotlight back on)

sudo mdutil -aE

(Reindexes all mounted volumes)

Spotlight appeared to see my HD and it's telling me it's reindexing as we speak. We shall see. I'll post back here when something happens. It might be a while if that's a progress bar I'm looking at because it hasn't budged in 20 minutes.

Well I let the reindexing go overnight and Spotlight is now seeing my files! Thank you to all who helped.