Answer
A sealed burrow is usually deliberate. Tarantulas close retreats for premoult, moulting, settling, moisture control or simply to remain undisturbed.
Do not open it
Do not dig through the plug or push food inside. A moulting spider is vulnerable to injury and live prey.
Water and moisture
Keep the water dish full and maintain the species' normal moisture routine outside the sealed retreat.
How long can it stay closed?
Some tarantulas remain sealed for weeks or months. The time depends on species, size, temperature and moult stage.
Emergency reasons to intervene
Flooding, collapse, chemical contamination or another immediate enclosure hazard can justify opening/moving the retreat. Curiosity does not.