There I nothing that can be done if investors decided to dump the coin and the price keeps falling. The only up option here is to buy back the token which could make the price stable but may take a long time for the price to recover from the bear trend. This has happened to many devs in the crypto market and what they could do is to buy back the dumped token so that the price can start going bull investors comes back to the project and buy them.
Marketing is one of the ways we can increase the numbers if investors of a token and keep the price going bull as more investors comes in to buy it.