They will only rise again if the coin that was dumped actually has some utility or reason to increase, typically for most good projects the value will return to somewhere close to the ICO price, though in this market I wouldn't be surprised if it doesn't even return to that. A lot of hyped projects now are actually trading massively below their ICO price, e.g. Pumapay trading at 1/10 ICO price which obviously sucks for investors, but it's great for those who believe in the long term potential of the project.