for now if your coin is still profitable then sell right now. if not profitable then hold your coin.
this btc decline is only temporary, btc i predict will be at peak in july 2018. so be patient
If you are there in the world of the crypto currencies since some good period and that you have a fair idea of how things work in the crypto world, then I dont think so that you will be surprised because of this downfall as this is a very common thing and that it will keep on happening in the future as well. You just need to hold your nerves when you experience any such thing as this will help you in the best possible manner.