if you setup a storage entry for "tank" that matches what you had before, you should see the volumes when browsing the storage in the GUI. then you can recreate the guest (using the same ID), and then run "pct rescan" (container) or "qm rescan" (VM) to add references to the found volumes to the guest config. you can then re-attach those unused volumes using the GUI. if the volume in question was your container's rootfs, then you likely need some manual work.