Is it in a container?
If so, the roots may have received some damage from the cold.
Or perhaps your temperatures are still on the low side; thus, the roots aren't yet working up to par.

The gray leaf likely died from cold or old age and just needs to be removed.

Can you post an overall image of the entire plant?

I see nutrient deficiency and lower right some fungal fruiting bodies, maybe obligate on the necrotic tissue. You may need to repot with fresh soil and fertilizer, and there might be a fungus hanging around as well so look around for that and treat accordingly.

Dan