If you removed a collection from a workspace, and are not sure where to find it, here's what you can do to recover it:
1. If you're on Postman Pro / Enterprise, and the collection was shared, you can head to "/team/elements?type=collection" on your team domain. Here, you'll see all shared collections, and you'll be able to "Share" them to the correct workspace.
2. If the collection was not shared, head to "/me/collections". Here, you'll see all your private collections, and you'll be able to "Share" them to the correct workspace.