If a token goes down after its launch, I'm afraid that there's no historical data to help us determine what the psychological support is. What comes to mind is whether you trust the project, the team, and its future use for the masses. If you didn't do thorough research or if you joined as a result of being hyped, then sell immediately.
Thankfully, Menlo One will be launching its decentralized ICO marketplace soon. With its help, even newbie investors won't get lost in terms of doing research since it incentives honest reviews & contributions from the community and penalizes fake ones for the purpose of creating hype.