Overwatering and root rot

What's happening

Calathea wants consistent moisture but not standing water. When the dense mix stays waterlogged, the fine roots can't get oxygen, suffocate, and begin to rot, so the plant can no longer hold its leaves up. They droop and go limp, the base may feel soft, and the soil can develop a sour smell.

How to fix it

Stop watering and let the soil dry down. Slip the plant out and inspect the roots — trim any brown, soft, mushy roots with sterilized scissors until only firm pale tissue remains, then repot into fresh, airy, well-draining mix in a pot with drainage holes. A peat-and-perlite or coir-based mix that holds moisture without staying sodden is ideal. Water lightly while it recovers, keeping the mix just moist.

This is general guidance based on common symptoms; individual plants vary.