Empties are key in Blender workflow, yet they often get missing. Here is a quick shortlist of common reasons why they may be missing from your scene:

• hidden in Viewport, or in Collection hidden in Viewport (eye)

• disabled in Viewport, or in a Collection disabled in Viewport (screen)

• in a Collection excluded from Viewlayer (checkbox)

• size too small, either in the Data tab, or in the Object properties (N-sidebar→ Scale)

• Show Overlays is disabled

• Show Overlays is enabled but Extras (under "Object") is disabled

• Empties are hidden in the Selectability and Visibility menu