Aviary canary suddenly died!?

i have 2 canarys 2 atiels and 4 finches, in a 6 by 3 foot aviary . both canarys were fine and the last time they were checked at 12 they were still fine. they were checked again at 3 and one canary was dead on the floor by the side of the aviary. i was so confused. there was no cuts or scratches on it and it is used to winter weather. inside the cage is seed water and grit that helps digestion. please help as i am very sad