It is probably one of two things…it got too cold. Anything below 60° F and they aren’t happy and can develop blackening leave. More likely is that you are keep it too moist. You can and should not water until the soil is dry right to the bottom of the pot. Always check the soil with a moisture meter or by poking your finger as deep in the soil as you can. If you even think it felt damp, wait a few days and check again. Overwatering is the #1 cause of houseplant deaths.