[100 Words is a story in 100 words or less, based off the word in the title.]
There is no defined point when you become lost. It is more of a dawning realisation that the place you are in does not seem familiar. That the trees and the grass and the roads are not what you expected.
If you were to pick a moment that really means to be lost, it is the moment of panic. The moment when you realise that there is nothing you know. Nothing to comfort you.
Nothing to tell you everything will be all right.