I don't know based on the rulebook. The provided context explains that when you sweep all three face-up Power cards, you "reveal three new Power cards from the deck" and that "they are immediately available for purchase" (page 6). However, it does not specify what to do if the deck runs out and you cannot reveal three new cards. The rulebook does not mention reshuffling the discard pile or any other procedure for this situation.

To answer this question, the rulebook would need to include instructions for handling the Power card deck when it is depleted.

Source: Core Rulebook (pp. 3, 4, 5, 6)