The DAO contract was attacked on 17th June 2016 where the attacker drained about USD 50 million worth of ethers using a recursive call vulnerability in The DAO contract code. Please find below how to get back your invested funds.
Ethereum Hard-Fork
Following the attack, Ethereum community, miners and exchanges leading the vote, decided to go hard-fork. After some code change, the hard-fork was completed on July 20th.