Root rot from overwatering

What's happening

When dumb cane roots sit in waterlogged soil they suffocate and decay, and that rot creeps up into the thick cane at the base. With no working roots the plant can't hold its leaves up, so the whole thing wilts even though the soil is wet. The base goes dark, soft, and mushy, often with a sour smell, and the rot spreads fast.

How to fix it

Act quickly. Unpot the plant and rinse the roots clean, then cut away every brown, soft, mushy root and any blackened cane with sterilized scissors until only firm, pale tissue remains. Repot the healthy portion into fresh, fast-draining mix in a clean pot with real drainage holes, and water sparingly while it recovers. As insurance, take a firm green cane cutting and root it — dumb cane regrows readily from a healthy stem section if the parent doesn't pull through.

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