Diagnosis
The internal mechanism that releases the cartridge door is likely jammed, often because an old cartridge wasn't fully seated or a bit of hardened detergent got in the way. Yeah, this one's common.
How to Fix It
Don't force the door. First, run a short, hot wash cycle (like the "QuickIntenseWash") WITHOUT any cartridge installed. The heat and water can sometimes loosen things up. After the cycle finishes, try pressing the release button again. If the door still won't budge, you can manually release it. Look for a small, recessed slot or hole near the cartridge compartment—there's a manual release there. Use a thin, flat tool like a small screwdriver or a straightened paperclip to gently push in and trigger the latch.
If That Didn't Work
The locking solenoid might have failed. You'll need to access it from inside the door. Unplug the dishwasher, remove the inner door panel (the screws are usually behind the door seal), and you'll see the AutoDos mechanism. Check for any obvious obstructions or a disconnected wire on the small solenoid that controls the lock.
Heads Up
When you insert the new PowerDisk cartridge, push it in firmly until you hear a distinct click and the status light on the door turns green. If it's not fully in, it'll cause this same jam on the next change.