Context clues. The prior sentence is talking about an asteroid that struck the earth. When the word “struck” is used when describing an accident, the effect is almost always a crash. An impact is when one thing forcibly comes into contact with another. A crash (for example: car crash) also when two things forcibly come into contact with another.
<em /><u><em>"She did not understand what happened."
</em></u>It is a complete sentence since it has a subject (she) and a verb (understand), and is independent (i.e. doesn't require another clause to make sense<u><em> </em></u>
