Currently I own a lot of altcoin, I bought them at ico. My account is being downgraded several times. There are many people who say bitcoin will continue to decrease, so altcoin prices also decreased compared to USDT. So now, should I sell them and wait for the price to buy back? Give me advice 
This dump is normal just like the green period these past few days. If you could, convert it to BTC if you want a better rock investment and then hold it for a while. Last year, BTC reached 20k USD. If you have BTC and that kind of peak was reached that's a good opportunity for you.