According to Microsoft Message Center:

We are aware of a temporary server-side issue causing Windows search to show a blank box. This issue has been resolved for most users and in some cases, you might need to restart your device. We are working diligently to fully resolve the issue and will provide an update once resolved.

Announcement Date: February 05, 2020 – 09:32 AM PT

Update:

This issue was resolved at 12:00 PM PST. If you are still experiencing issues, please restart your device. In rare cases, you may need to manually end the SearchUI.exe or SearchApp.exe process via Task Manager. (To locate these processes, select CTRL + Shift + Esc then select the Details tab.)

Update Date: February 05, 2020 – 12:00 PM PT

Source: Windows message center