An overflow box should maintain the siphon. The image below, courtesy of Melev's Reef, shows a HOB
1. The water flows from the display tank into the leftmost chamber.
2. The siphon pulls the water up and over the tank wall into the center chamber.
3. The center chamber overflows into the rightmost chamber and down into the sump.
When the power is cut, water stops flowing into the first chamber and the water levels in the first chamber and second chamber will equalize , keeping the siphon tube full of water. The third chamber will drain completely.
When the power is restored, water flows into the first chamber again. The siphon is still intact, so gravity will cause water to flow through the siphon tube into the second chamber and overflow into the third chamber and down into the sump. No actions needed.
Here's all the sump information you'll ever need:
Melevsreef.com | Acrylic Sumps & Refugiums