I’m new to Blender but I’ve been modeling a speed boat. I was making a little LCD screen for a GPS to go on the boat, and I created a cylinder. But when I zoomed out, the boat was an object, and the cylinder was an edit object. I couldn’t edit the boat, only the new cylinder. I had to start again from scratch.
Be careful what mode you are in object vs edit (tab key). You created your cylinder while in object mode so it’s a separate object. You should have created it when your boat mesh was in edit mode so it would be part of that mesh.
No need to start over -you can easily join this object with your boat mesh. Select both in object mode and Ctrl-J