I’m very sorry about your loss. There could be many things that caused this. The only way to know for sure is to have a Vet do a necropsy. Birds are good at hiding signs of illness, so he may have had something serious going on internally. It’s possible that something scared him in the night and he fell, cause him to die. Dying birds often twist their head back and also tend to die with their eyes open. Those are not signs of anything in particular. One of the main causes of sudden death is a toxin – particularly something airborne. Birds are sensitive to any strong fumes or odors, and even something that doesn’t harm a person can harm or kill a bird. Many things used around the house like the plug in air fresheners, cleaners, candles – anything that emits a fume/odor. If there was something in the air, by covering the cage, it will concentrate the fumes and kill the bird. This has happened to many birds when the owner buys one of those mite protectors for the cage and then they cover the cage at night. If you had anything cleaned like carpets, any type of exterminating for bugs, a self cleaning oven – all of these thing can be deadly for a pet bird. Again I am sorry this happened.