I guess they are gone forever.
Yes, they are gone.you were hacked. Your system is compromised .
I would format all computers/smartphone that you ever used to access your wallet.
Where did you store the seed? Was it in a paper? Or in a Google draft, drive, cloud storage?
Blockchain.info wallet is not very safe, as there are many ways a hacker could get access to it.
Maybe even the e email that you used to create the wallet is compromised. Change its password and add 2fa to it.
The 12 word seed was only stored on plain paper and the papers aren't lost or stolen.
I was using a secure mail provider, Tuta.io and 2FA was enabled on both the email account and the blockchain.info wallet.
I know i was probably compromised but I have no idea how. All the latest updates of Ubuntu are installed and no new software has been installed in the previous 2 months or so.