What wallet were you using?
I'm using Bitcoin Core 64 bit. (now v0.9.2.1 - QT ver. 5.2.0)
Did you have it password protected?
If so was it a long, random password or a short common one?
Protected with passwd. 8 characters (each different, some lettes, some digits...etc.).. that's why I dont know how this could happened.
Relatively weak and easy to crack assuming someone got your encrypted wallet.dat file and could set it up in a dedicated machine to brute force it. Can you think of how anyone might have gotten your wallet.dat file?
(I use 32 character random passwords generated by LastPass).