Which leaves are affected? Are they the bottom leaves only or all over the plant? Most of the time if it is only the bottom then it is root rot. If so,

1. take the root ball out of the pot

2. remove all affected leaves and rotted roots

3. place the plant in NEW dirt in a pot with good drainage

4. water slightly less than you did before

if it is all over the plant, it is most likely under watering. start watering more frequently.

I really hope this helps and just comment if you have more questions!!