Diagnosis
The unit isn't getting power from the adapter, or the internal circuit board is fried. A green light on the wall plug just means *the plug* has power, not that it's delivering it correctly to the filter.
How to Fix It
First, check the other end of the power cable where it plugs into the port on the side/bottom of the AQ-5200 unit. Ensure it's fully seated. Unplug and replug it. Try a different outlet. The unit itself should have a small LED illuminate immediately when powered.
If That Didn't Work
The low-voltage DC adapter might have failed. It's a standard 12V DC, center-positive plug. If you have a multimeter, check for 12V output. If not, contact Aquasana for a replacement adapter—it's a common part to fail.
Heads Up
Without power, the electronic shut-off valve will eventually close and block ALL water flow through the filter as a safety measure, so you'll get no water from the filtered faucet.