"Why are you troubled?"
This was a nice explanation I thought:
困ってる means the subject is presently or actively troubled about something. It emphasises the state of being troubled.
困る emphasises the speaker's anger or annoyance. (困るよ or 困るんだよ)
困った can be used when you realise you're in trouble ('uh oh' or 'oh dear' - 困ったなぁ), or in a similar way to 困ってる."
It's a very subtle (and confusingly vague) nuance that comes into play with reference to psychological circumstances. But the difference goes something like this (not the real translations btw, it's the conceptual translation): 1.なぜ困りました - 'Why did something lead to you being troubled?' 2. なぜ困っていますか - 'Why are you currently in the state of being troubled?'. Japanese is fun :D
That sounds more like "why will you be troubled?" When using a verb to describe your physical or mental condition, the past tense (困りました) or the present progressive (困っています) are used to tell how you currently feel, while the non-past form (困ります・困る) talks about how you will feel in the future.
If you check the Pera Pera Penguin link (http://www.japaneselunch.com/peraperaPenguin/pdf/ppp026.pdf), Hitomi Hirayama explains the difference using the verb 疲れます (tsukaremasu, tired) as:
I'm tired. (I feel ... at that moment)
I have been tired (since...) (the condition continues)