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 
If you don't have to cash out tomorrow, keep calm. You may trade a bit or hodl or buy, do your crypto business as usual. Just don't panic. You all are pros, you all have invested only the money you can afford to lose. So what.