It was supplied over Lake Ladoga when the ice froze, but unfortunately the ice froze later in the year than usual and thawed earlier than usual during that first winter. It was dangerous stuff when the ice was thin, but when it was thick you could drive a tank over it, and they did.

Later on they were able to partially lift the siege in the south east just below the lake and could get some supplies in by train.

Of course it's always possible to get small quantities in, but to feed a city of millions....

It would depend on there already being beach landing craft in the lake when the encirclement happened. There weren't any other major ports on the lake at the time other than Leningrad and Shlisselburg which the was on the German side of the front and heavily contested. The German line also went to the Southern bank of the River there up until it turned North West to get into the city so you couldn't use that so the options for bringing food in via the lake were pretty limited and any losses would be effectively permanent until the siege was broken.

https://upload.wikimedia.org/wikipedia/commons/9/90/Leningra...

https://upload.wikimedia.org/wikipedia/commons/thumb/9/97/Si...

https://64.media.tumblr.com/8fcb0d31607d4cd05fbe82998afc91ef...