There's something called the 51% attack.

The attacker generates a blockchain that's larger than the real blockchain. That prevents any transactions being confirmed. To do this the attacker needs as much computing power as the rest of the Bitcoin economy, and they need to want to spend money to destroy Bitcoin. (Because they'd make more money either mining or double spending).

As much computing power as the entire history of computing power spent on the block chain. They don't just need to exceed the amount of computing power working on bitcoin today, but all of it that has been put into bitcoin since it was started, put together.

Even if that did happen, I think bitcoin could recover. All you'd need to do is get everyone to agree on a point in time where the blockchain was untarnished and to reset to then. That would be easier than it sounds, given the small number of existing clients. It would be damaging, but only in the short term.