Your bones were hidden. To unhide them, once in Edit mode press ALTH and your bones will be back!
Hidden Bones are only Hidden in Specific Modes
It is worth noting that bones hidden in one mode may be visible in another mode, and vice-versa. Here is an explanation from the Blender Manual:
Note that hidden bones are specific to a mode – i.e. you can hide some
bones in Edit mode, they will still be visible in Pose mode, and
vice-versa. Hidden bone in Pose mode are also invisible in Object
mode. And in Edit mode, the bone to hide must be fully selected, not
just his root or tip…