The eruption of Mount Tambora was a huge one, and the amount of ash it released into the atmosphere managed to affect the whole world in the coming year and a half to two years, depending on the place.
The ash in the atmosphere was slightly reducing the amount of sunlight that was reaching the Earth, and because of that the temperatures were lower. The biggest problem was occurring during the night because the temperatures were going significantly lower than usual.
The crops were suffering very badly because of the sudden change of the climate, so big portion of them were either dying or producing very small amounts. That led to a famine, and around 80,000 people died from starvation.