Fungicide will not help root rot. Root rot is caused by an overabundance of standing water or moisture within the pot. Unfortunately, damage caused by root rot is irreversible, you can, however, take steps to try and prevent further damage by drying out the soil. If there is any standing water in the plant's liner drain it out, do not attempt to water again until the soil has dried out. There is a slim chance remaining undamaged roots, if any, may survive, and new roots will generate.
A severe case of root rot is often fatal, so if this comes to pass just be careful about overwatering, and in the future, make sure your plants are potted in a manner that allows water to drain.