1) Unpot the plant, rinse all the soil off the roots and repot in fresh
growing medium.

2) Identify the plant and figure out what you need to provide to get the
plant to grow properly; change those factors (including soil) once you
figure it out.

The plant may have picked up a fungal or viral disease you're just not
seeing, and may still die. But in my experience, repotting has pretty
good success as a last-ditch effort to save a potted plant that you
don't know what's wrong with it.

Kay