Hacked bitcoin account.
Bitcoin transactions cannot be reversed - unless you are sending coins to someone you know by mistake and they are willing to send it back - which in itself is also relatively unlikely.

If someone has sent money from your account without your knowledge, then they have got hold of your private keys. What wallet did you use? Is it local on your machine? Is it online - in which case you may have not set a complicated enough password.

Sorry about your loss but you need to make sure your wallet is secure in order to prevent this from happening again.