Completed journeys don't disappear, they're still accessible and can be reopened at any time. Here's how to find them and reactivate one if you need to continue working on it.
Finding your completed journeys
Go to the Grow tab and scroll down to the Journeys box
Select View All
Use the filter to switch from Active to either All Journeys or Completed Journeys. Your completed journeys will now appear in the list.
Re-opening a completed journey
Before you start: You can only have one active journey at a time. If you currently have an active journey, you'll need to archive or delete it first before reopening a completed one.
Filter your Journeys to Completed Journeys
Click into the Journey you want to reopen
Select Reopen in the top right corner
Your journey will be reactivated. Filter back to Active journeys and you'll see it listed there, ready to continue.
Video tutorial for a full demonstration: