Were you actually in hospital (or demonstrably incapacitated) during the one month period after you were sent the tax form? If not, I don't see how that is relevant, and if you were then the appropriate course would then have been to apply for an extension at that time, which I'm sure would have been granted. If you had, for example, been on an extended trip abroad with no-one attending to your mail, and had contacted IRD immediately on your return to explain the situation then I think that too would have been OK.
I think the moral of the story is not to ignore communications from HK bureaucracy. In my experience, if you play by the rules and do everything they ask then you tend to get good results. If you don't then you now see the outcome.
Sorry to be blunt, but by your own admission you simply "forgot".