Have network DDoS attacks ever been successful on any coin? Has a cryptocurrency network ever been fully DDoSed so that the blockchain did not advance during the attack?

If so, which one and how?
This is more or less impossible, since you don't know where the miners are. Attacking the major mining pools is feasible, though they deploy some anti-DDoS measures. But even when such an attack would succeed, there still are people non-affected (eg. running behind NAT) and running the *coin network like if nothing happened.

The only issue that I can notice there is that the difficulty for getting a new block would be too high, so that the blocks would be mined much slower.