Short Film: After the Sun went out, the earth became a cold, dangerous and dark place and people avoid leaving the house in fear of the temperature, murderers and lack of oxygen. When a young girl, depressed by her parents disappearance, realizes her little sister has left, she must go out and find her before it’s too late.

What do you think of this:

"In a world with no sun, a girl obsessed with her parents disappearance must face her fear of the dark ?to rescue her sister from a cult who intends to sacrifice her to the Sun god."

Understood, thank you for all your help.

I like your re-vamped version. It paints a pretty good picture for me.

The beginning of your logline delays the start of the story by having this: "In a dark and dangerous world with no sun and most of the people gone," before it tells what happens.

Better to jump into the action sooner rather than later because the point of the logline is to describe the plot above all.

So if you were to remove the first sentence it would read:
"A terrified young girl must find her sister before she dies from the cold, lack of oxygen or gets killed by survivors gone mad?

However this is confusing because the environment is not mentioned and there is no inciting incident.
Also the MC flaw is too generic and therefore does't relate to the specific obstacle she will face It begs the question; terrified of what?

Her depression is not a flaw it is a condition and a result of her circumstance I would expect most people to be depressed if they were in her shoes? It is nothing she can necessarily over come and I think pales in comparison to her over coming for example: nytcophobia or agoraphobia.

In essence the lack of oxygen, sun and general danger should come across from the setting description as it plays such a heavy part in this concept but not elaborated on. Let the description engage the readers imagination and make them want to know more rather than specify exactly the nature of this dreadful place.

I think Richiev gave a good re draft bellow the brevity in the setting description helps the reader understand the stakes. I would recommend perhaps an even shorter description such as: post apocalyptic, but that comes down to personal taste. As long as it sets up a setting that could have all the needed "bad things" to make MC's goal harder (and therefore more interesting) to achieve.

Just too bloated, especially for a short film.

"A young girl ventures forth into the darkness to rescue her sister from murderers lurking in Earth's eternal night."

OK, pretty vague. It's not great. But you can definitely cut your logline down to something tight like that.

Also -- "YOUNG" is not a great descriptor for a character. "GIRL" gender is a stupid function for a character to have. Don't define your character simply by being a 'young girl' ... or else they're bound to be paper thin. Is she optimistic? Naive? Rambunctious? What else defines her beyond being a girl? (Also, gender is identified in the pronoun you use ... "her" ... so you're wasting words in specifying that she's a "girl" anyway).

"When her sister disappears, a teen with nytcophobia must brave a world stricken with darkness and survivors gone mad if she's to locate her sibling and bring her to safety."

Nir Shelter, I agree with you.

So what do you think of this log line:

"In a dark and dangerous world with no sun and most of the people gone, a terrified young girl must find her sister before she dies from the cold, lack of oxygen or gets killed by survivors gone mad"

Also, just for argument's sake, how could I, in your opinion, transform this so as to maintain depression as a flaw ?

I think the story elements are good only the flaw needs to relate to her obstacles as her over coming her flaw should be a condition of achieving the main goal.

My suggestion for clarity sake would be to change her flaw to only fear without the depression. The logline can then change to:
After her little sister has wondered alone into the post apocalyptic wasteland they live in, a young girl petrified of the outdoors must find her before any of the surviving gangs do.

Precisely "when that started to happen" But in your logline you state "when the Sun went out so then it's much too late. You must rephrase your logline.

Your story element are ok.

But these people would literaly live in an ICE AGE; the last two Ice ages humanity has servived but this Ice age would be much more severe. Only with superiour technologie humanity could survive. But do we still have this knowledge? If there would be lack of oxigen I don't know?

It all lets me think of my favorite science fiction writerJack Vances' The Dying Earth tales (that in reality would not be illogical) :

"A dim place, ancient beyond knowledge. Once it was a tall world of cloudy mountains and bright rivers, and the sun was a white blazing ball. Ages of rain and wind have beaten and rounded the granite, and the sun is feeble and red. The continents have sunk and risen. A million cities have lifted towers, have fallen to dust. In the place of the old peoples a few thousand strange souls live. There is evil on Earth, evil distilled by time ? Earth is dying and in its twilight." The "few thousand strange souls" are "feverishly merry, for infinite night was close at hand, when the red sun should finally flicker and go black". There is magic, but we learn it is somehow derived from "a strange abstract lore ? termed 'mathematics' ? Passive itself and not of sorcery, it elucidates every problem, each phase of existence, all the secrets of time and space. Your spells and runes are built upon its power and codified according to a great underlying mosaic of magic."

For inspiration read: http://www.theguardian.com/books/2011/dec/05/fantasy-tales-dying-earth-jack-vance