Once a sequence has sent out all the steps and everyone in the sequence has been marked as Finished, it's not over yet.

If you would like to continue with the sequence, you can easily add a new step:

Once a new step has been added, the next step is to change the Finished status to Active for those contacts in the sequence who never replied, bounced, or opted out. Use the smart filter to select all such prospects and click Apply. You'll see a list of people for whom you need to change the status:

Now, change the status of all selected people by using this option:

Once those prospects are active, the sequence will resume and the prospects will move on to the next step you created. Also, make sure the sequence has a green toggle, which means it's ready to go!