Hi
The Redo and Undo option arrows will only step back/forward in the order the timeline was processed.
Obviously things like transitions and effects can usually be removed and the timeline restored. Deleted sections can be restored by dragging out the clip ends with the white brackets but this might impact on the position of other clips on the timeline. A lot depends on the complexity of the timeline, but you can usually get it back with a bit of care.
Nat