One assumes an explosion is unexplained unless it is stated otherwise. Delete unexplained as an unnecessary word.
Spell out numbers instead of using numerals: three, not 3.
The em-dash is wrong. Replace it with a period. (That’s a full stop, for those of you who speak UK English like the creator of this example.)
Always capitalize the pronoun I.
Without a comma after devastation, the sentence says the devastation itself is holding the crisp packet. I’m quite certain that’s not what the writer meant. 🙂
Three minutes ago, I was watching TV and eating snacks when there was an explosion outside. Now I’m staring at total devastation, holding a crisp packet full of dust!