DIAGNOSIS The plastic latch mechanism on the filter compartment lid has become jammed with mineral scale or the locking tab has broken.
FIX Apply a few drops of white vinegar around the edges of the lid. Let it sit for 10 minutes to dissolve scale. Use a thin plastic pry tool (like a guitar pick) to gently work into the seam and release the latch. Do not use metal. Once open, clean the entire compartment and lid thoroughly.
IF THAT DIDN'T WORK The plastic locking tab on the lid is likely broken. Inspect it. If broken, the lid will not latch securely. You need to order a replacement filter holder assembly from Cuisinart. The lid is often part of the whole holder.
HEADS UP You can run the machine without the charcoal filter. Just ensure the red bypass plug is inserted in the filter holder. Running it with an empty holder but no plug will cause water to spill inside the machine.