I’m using RW8 Version 8.4.1 (20817) using Stacks 4. I restarted RW and the column showing the stacks is missing. I followed some suggestions from the community and forum but still had the same problem. I’m running High Sierra 10.13.6. on a late 2015 iMac.
If the Stacks Library is hidden from view, then just click the Library button to show it again. Make sure that your RapidWeaver window is expanded large enough to display the Stacks Library.