I'm new to Blender and I'm fumbling around a lot with it. I created this shape from a cube I turned into a rounded shape by applying a Subdivision modifier. At first, I could see all its vertices, but somehow at some point, while I was editing it, its mesh became oversimplified to only a few vertices and now I'm only able to edit its extremities. How can I access the rest of its vertices?

[![enter image description here](https://i.stack.imgur.com/i77oF.jpg)](https://i.stack.imgur.com/i77oF.jpg)