Is there a way to restore live corners in a shape? For example when I expand a text or object, sometimes the symbol of dynamic corner doesn't appear.
The corner widget has a hard limit of 100 handles of the current selection. When you expand your text, if it generates more than 100 handles, the corner widgets will not appear. You can select a few letters at a time and it should work as normal, so long as the number of selected handles does not exceed 100.
I think you are referring to live corners.
In that case you just need to go under the
View menu and down to
Hide/Show Corner Widget
I would also recommend checking out Adobe's site on the topic