Updated June 15th, 2019.
If you want to use Spotlight to search for something, don’t bother clicking on the magnifying glass at top right. Just press ⌘-Spacebar and Spotlight pops right up. If it doesn’t, go to Apple menu/System Preferences/Spotlight, then Keyboard Shortcuts… and make sure the preferences are set that way. See below:
Yes, that’s the Keyboard preference pane, so you could have gone there directly, but going to Spotlight’s preferences first means you land in the right place in the Keyboard preferences.
(Thanks, Rick, for pointing out that the preferences might not be set to ⌘-Spacebar.)