I agree with you, panic always lead to wrong decision. Keep calm. After that, Look back your coin, if it's a good coin, you should hold or even buy more coins with low price and wait for recovery. You will get a big profit. But if your coin is not good or it has a bad new problem. You should sell immediately. 
So we need to research all information of coin which we want to buy before, this is very important, it will help choose a good coin and keep calm when the price of market go down. Many newbies who usually skip this step, will panic the market go down. For me, i usually put goal the time hold coin which i bought so i'm never Panic when the price of coins is go down.