I apologize that no one has responded to your question sooner. Different experts come online at various times. I just came online and saw your question. My name is ***** ***** I’m a biologist with a special interest in avian health. I'm so sorry for your loss of Toby.
There's no way to be certain of the cause of his death without having a vet do a necropsy - and even that doesn't always give certain results. However, when a bird dies suddenly, without showing symptoms, there are several more likely causes. One is exposure to a toxin. Birds are very sensitive to artificial fragrances, such as those found in scented candles and air fresheners. The fumes from nonstick cookware can cause death in birds, too. And even a slight exposure to pesticides and cleaning products can be fatal.
However, the most likely cause of death is a heart condition. Birds can have conditions, such as cardiomyopathy, that cause no symptoms, but suddenly the heart just stops beating. It's similar to what we see when a young human athlete dies suddenly on the field. Stress or fear can be a factor. If Toby already had a heart condition, you're yelling at him may have complicated it, but it wasn't the direct cause.
Another cause of sudden death is that a bird may become frightened and fly headfirst into the side of the cage or a wall. That can result in a broken neck or head trauma. A siren outdoors, a car horn, a backfiring engine, and many other things can trigger such fear.
Most other causes of death would have presented with some symptoms - breathing difficulties, appetite loss, fluffed up feathers, etc. Since you saw none of that, the problem was most likely in his heart. Sometimes, though, illness symptoms can be so subtle that a bird owner doesn't notice them.
You asked about forgetting to give Toby water. If you forgot for only one day, that wouldn't cause a big problem. If it went on for longer than that, it could cause severe dehydration, which can lead to weakness, seizures, and eventually death. I hope that wasn't the case.
If you have more questions, just let me know. I know words don't really help, but all I can do is offer you my sympathy. I hope that in time your good memories of Toby will replace the pain you feel now.
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