For some reason your vertices are no longer assigned to any vertex groups, so the armature can't control them anymore, there may be a way to assign them back to the right vertex group and switch the armature to rest pose without messing the mesh, but what you can do is:
First reset the rotation of the armature AltR, switch it to Pose mode, CtrlA > Apply Pose as Rest Pose, parent the character to the armature With Automatic Weights, give it the default pose you want, apply as rest pose again. You can symmetrize the mesh and also the armature.