Happening for the past few months lately. I had to look for a solution other than rebooting the PC or restarting Windows Explorer. It appears to be some bug in indexing.
Solution: Windows Key + R, type command: msdt.exe -ep WindowsHelp id SearchDiagnostic
This opens up the Search and Indexing Troubleshooter. I then clicked on "Advanced" to reload this troubleshooter in Admin elevated mode. Then when prompted with a list of reasons to troubleshoot, I selected the first one: "Can't Start a search or see results", after finishing this process, it is fixed.
Solution Source: Microsoft (Solution #2)