I don't think there's such a thing as de-evolution, or at least there's not any evidence of this sort of thing: going back to exactly as it was. But an organism can continue to evolve into what would appear as reverse. For instance dolphins, whales, orcas, etc have evidence that they were once land animals that went back to the sea (where all organisms started).
If DNA can mutate, it should be able to mutate into things that are beneficial and things that are not beneficial. Evolution says the non-beneficial mutations don't make it since only the strong survive. However, if a human de-evolved, meaning went back to the chimpanzee family, it could survive. Therefore, if evolution is plausible, de-evolution must also be plausible. Why isn't it seen? Simple mathematics says it should be there. There should also exist any species in between, as it wasn't like that species wasn't fit for life.
A few things:
1. "Only the strong survive" is completely false. There's a lot of luck involved. For instance, Neanderthals and Humans once co-existed. By all accounts Neanderthals were the superior species. Humans filled the same niche as them and when that happens only one species can win out. Scientists have long wondered how/why humans won that "battle". The most recent evidence suggests it was merely persistence. More and more humans kept migrating from Africa into Europe and over enough time, the Humans won out because they just had more chances to do so.
2. We did not evolve from chimpanzees. Chimpanzees and humans have a common ancestor that is long extinct, but we did not evolve from them.
3. Let's say somehow, someway, this "de-evolution" could happen... how would we see that? Evolution happens over thousands... millions of years. If it were in the fossil record we would say that a species was around before we previously thought, or lived longer than we had previously thought.