Hey @nathanagin43! We have a guide here192 to help you determine if your subscribers will receive newly added emails to a sequence. If a subscriber is marked as 'Completed' for a sequence, you can also reach out to our team at firstname.lastname@example.org (or via the in-app messenger in your account) letting us know their email address and the name of the sequence, and we can re-activate them for you. (We can also re-activate all the completed subscribers on a particular sequence, if you want — just let us know!)
As for visual automations, if a subscriber has reached the end of an automation, then they will not be affected by any additional steps added to that automation because they already are marked as completed for it. If you need to create a new automation and pull existing subscribers into it retroactively, we have a guide on that here106. We also have a tutorial here95 on how to make changes to an automation without interrupting its current subscribers, if that helps.
If you have specific questions about particular automations/sequences in your account, please feel free to reach out to our team and one of our Product Specialists can take a look :)