if your funds were stolen from multibit it has nothing to do with multibit.
It means your computer is compromised. Using qt OR blockchain.info
on the same computer will make you lose more money.
But my computer was not even on.
What if the hacker had access to the wallet backup and the password? It wouldn't need my computer that way, right?
One way or the other, the attacker got access to your wallet file and the password. Whether that's from your backup or from your main machine is unknown. Note that if your computer has been compromised, the attacker could empty the wallet at any time, even when your computer is off.