How can I delete lines from cylinders without deleting the geometry?

If you deleted him from the model space and nothing more, you can get him back by going to the Components window and choosing In Model (click the house icon.) He’ll be in there. Select him and drag him out into the model space. No need to open another copy of SketchUp.

That’s one of the benefits of using components. Deleting them from the model space isn’t enough to make them gone forever.