DIAGNOSIS Coffee grounds or mineral scale have jammed the locking lugs or the rotation mechanism inside the brew head.
FIX Pull the basket out. Inspect the two black plastic lugs on the underside of the brew head. They should move freely. Using a damp cloth, clean all coffee residue from these lugs and the grooves they rotate in. Then, look at the basket itself. Ensure the red silicone seal is clean and properly seated in its groove on the basket.
IF THAT DIDN'T WORK The internal spring that provides tension for the lock may be broken or dislodged. This requires disassembly of the brew head housing to inspect.
HEADS UP Forcing the basket to lock can crack the plastic lugs. If it doesn't turn smoothly with moderate pressure, stop and clean it again.