The bitcoin network has never been hacked. Individual coins that have been poorly secured, however, have been stolen, and are always at risk unless proper security measures are taken.
The incredible amount of computational power devoted to securing the bitcoin network makes it unlikely that it could ever be victim to a sustained attack that would make the network unusable.
That being said, individual bitcoin wallets have been hacked many times in the past, usually due to poor user security.
Using strong, redundant security systems such as offline (cold) storage and multisig technology greatly reduces the risk of theft of your coins.