I've been brewing this cider for 8 days. It was going smooth and had a lot of krausen so I took out a bit two days ago, but now it has passed more than 48hh and I was expecting some more krausen or some more movement, but even the airlock has nearly stopped bubbling now.
Did I kill my cider? What should I do?
(I have a 6L plastic carboy, and used 3,5g of champagne yeast(bioferm champ))
No. You didn't kill your cider, this description sounds like a perfectly normal fermentation.
Generally a yeast fermentation is vigorous in the first 1~7 days (typically producing a krausen), this phase is known by the term "primary fermentation". The time taken can be significantly different depending on temperature, sugar-concentration, amount of yeast, etc. etc.
Once most of the sugars have been consumed by the yeast, the process starts to calm down a bit. The krausen collapses, dissolving back into the ferment - sometimes you're too busy to see the krausen, just a crusty ring let's you know it's been and gone.
At this point, in the cider there may only be a small amount of sugars remaining, and/or the more complex sugars - yeast always processes the simplest sugars first. Although generally juice mainly has simple sugars (unlike say beer wort).
This next stage is known as "secondary fermentation". Although not rapidly consuming sugar, the yeast is still working. It's settling out, preparing to go dormant. Importantly part of this is re-processing some un-tasty fermentation by-products - this is why you always want to ensure your fermentation is complete. So don't go bottling your cider as soon as the krausen drops!
If you've not been measuring gravity with a hydrometer (AKA "saccharometer"), I would let the yeast continue until 2 weeks have elapsed. However this timing assumes the the ferment is relatively warm at 16-20C. Below 16C some yeast slow their work dramatically, so it's hard to give anything other than approximate "usually" timings.
So wait ~4 more days. You may see the cider starting to clear as the yeast drops out of suspension. Then measure the gravity, confirm it's finished and go from there. If you're not measuring gravity, bottle into PET bottles rather than glass.
