If the campaign has already been completed, then the email won't be sent anymore.

The emails in Prospect Reach follow a sequence

Let's say your campaign originally has 2 emails which are:

1 - Opening email

2 - Followup email #1 to be sent 1 day after the opening email

Then after 3 days, you added a new follow-up email #2 to be sent 2 days after the opening email. Since 3 days have already passed since the opening email was sent, follow-up email #2 wouldn't be sent anymore.