This sentence comes from the opening scene to the visual novel “Clover Days”

どうして、みんな泣いてるの……？

Vocab

どうして – Why; how

みんな – Everyone

泣く – なく – To cry

This sentence is pretty short and easy to understand, but it does show a useful contraction that you should be aware of.

どうして、みんな – depending on the context, どうして can mean either “why” or “how.” Here it is clearly “why.” みんな simply means “everyone,” and you can see it written in Kanji like this: 皆. So this means: “Why, everyone”

泣いてる – This is the verb to cry in its te-form with a contracted いる. This is a common thing that you will see all over the place. Instead of typing or saying いる, you just use る. The same goes for the polite form います → ます. So this in polite form would be 泣いてます

The の on the end is a particle that shows a desire for explanation. The speaker is wondering why everyone is crying and wants some sort of explanation.

This sentence comes out to mean: (with a nuance of seeking explanation)

Why is everyone crying?