Someone asked the question, "What is the proper usage of the word tragedy?" And my smartypants answer is, "Well, it depends on what you're talking about at the time. Is the feeling gone and you can't go on?"
A more considered answer lies under the cut, together with examples from The Sentinel and Stargate.
Let's start with some definitions, from the Collins English Dictionary.
Tragedy:
1. (esp in classical and Renaissance drama) a play in which the protagonist, usually a man of importance and outstanding personal qualities, falls to disaster through the combination of a personal failing and circumstances with which he cannot deal
2. (in later drama, such as that of Ibsen) a play in which the protagonist is overcome by a combination of social and psychological circumstances
3. any dramatic or literary composition dealing with serious or sombre themes and ending with disaster
4. (in medieval literature) a literary work in which a great person falls from prosperity to disaster, often through no fault of his own
5. the branch of drama dealing with such themes
6. the unfortunate aspect of something
7. a shocking or sad event; disaster
In literary discussion there seems to be a bit of bun-throwing over the exact nature and definition of tragedy, which is outside the scope of this community. In day-to-day speech, tragedy and tragic are essentially used in the meanings of definitions #6 and #7 above.
Many readers of this community are passionate about some aspect or other of language, and for some people the 'debasement' of tragedy into a word that refers to any sad event is a pet peeve. That application does seem to lie inside most of the definitions to be found on the internet. However, the more formal use of tragedy still tends to have the nuanced meaning of sad events that are outside 'normal' expectations. That meaning comes out of the dramatic use of the word, presumably, and I certainly prefer usage that reflects that nuance. Someone dying of old age and associated illness would be sad but probably not be regarded as a tragedy, whereas the death of a young person or child is more likely to have tragic associations.
Blair didn't think that a squashed Wonderburger was quite the tragedy Jim's expression made it out to be.
Many marriages are undermined by the tragedy of a death of a child, and Jack and Sarah O'Neill's relationship was no exception.
If you declare a top-model's broken nail a tragedy in fanfiction, readers will hope that you're using hyperbole for humorous effect or looking to make a point about the character who's making that declaration. Overall, tragedy refers to events that are, at the least, sad, and potentially overwhelming or even catastrophic.