Use Repair mode to reset broken tangent handles to an unbroken state.
To reset broken tangent handles:
In the Edit Mode option box, select Move to select the broken keyframes you want to repair. You can click the Select All button to select all keyframes.
(a) Edit Mode option box(b) Return toggle button
In the Edit Mode option box, select Repair.
All broken tangent handles for the selected keyframes are restored to an unbroken state.