Bitcoin transactions are irreversible, so you can't get them back
But if you know HOW it happened you can prevent a future occurrence.
How do you usually access your wallet? Do you use the website on a PC? Or a mobile app?
Were you given a seed phrase (a set of 12 or so words)? How did you store the seed?
If you're using a PC then you should do a full scan of it for malware.
It's blockchain.info, it's non-seeded so he doesn't have the 12 words.
If your funds actually exited your wallet there is nothing you can do, I assume that there is nothing to do with blockchain.info operators and it's probably your wallet/computer has been compromised.
It's a lesson to learn- next time if you store funds that you can't afford losing then do it on a hardware wallet offline, so you won't risk losing your funds due to lack of security.