I use Spotlight more than I like to admit, usually for launching apps. Recently, some of my apps and files stopped appearing — even ones I access frequently.
After sifting through numerous posts online, I finally discovered one that seems to work. Running this command in the terminal seems to do the trick:
sudo mdutil -E /
If you're having problem with just a certain disk, you can replace
/ with the appropriate device name (tip: drag the device into the terminal and let macOS input it for you).
Finally, my Spotlight is back to normal!