Overwatering or root rot

What's happening

A fern wants steady moisture, but it still needs air at the roots — soil that's kept soggy with no chance to breathe causes the roots to suffocate and rot. The plant responds by collapsing whole fronds, which yellow and then brown from the base outward, often worst in the dense middle of the crown where air can't circulate. A sour smell from the soil is a strong sign rot has set in.

How to fix it

Check the pot and roots. Let the surface dry slightly before the next watering and make sure the pot has open drainage holes and isn't sitting in a saucer of water. If the soil smells sour or the roots are brown and mushy, unpot the fern, rinse the roots, trim away every soft brown root with clean scissors, and repot the healthy portion into fresh, airy mix in a pot with drainage. Thin out dead and collapsed fronds from the center to restore airflow, and water to keep it moist but never waterlogged.

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