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....