A “lost” person is someone who doesn’t have a personal relationship with Christ, but lives in a place where they have access to scripture in their language, churches, believers, community groups, Bible studies etc. In other words, they are within relatively easy access to the Gospel.
An “unreached” person is someone who doesn’t have a relationship with Christ, and has little to no access to scripture, churches, believers, Bible studies, community groups etc. Chances are they not only do not know a follower of Christ, but they also do not know anyone who knows anyone who is a follower of Christ. In other words, they have almost no access to the Gospel.