Why my monkey is rendered with only a mesh? I'm totally noob to the blender world, and I accidentally made my monkey like above.
the monkey was originally a solid not mesh. I managed to find the way for restoring my monkey to solid again in the dropdown list at the Object > Viewport Display > Display As. However, I wonder if there are some shortcuts to do this.
If you right-click that dropdown you can assign a shortcut key, but personally I don't use this feature very often, so I've never set one.

The usual way is to use the Z key to change the viewport rendering mode (note this is for all objects in the viewport).  You can also use the buttons in the top right of the viewport.