In the NLA editor when you convert an action to a new strip then the keyframes disappear, is it possible to "get them" back?
The action editor and the graph editor both allows us to edit the active action of an object, only one action can be active for an object at any given time. When an action is added to the NLA editor it is unlinked from being the active action for the object.
While an action is removed as the active action for the object, it is still an action which contains the same editable keyframes and f-curve data, it can be edited again at a later time by making it the active action again. This can be done by selecting it in the action editor when the object is selected.
You will also find that it can also be associated with a different object and added to the NLA for both objects allowing both to be animated the same with the timing being offset for each object if desired.
The NLA Editor is designed to deal with a 'higher form' of animation than usually necessary so it automatically removes any actions from Action Editor.
Press Tab or select 'Start Tweaking Strip Actions' from the Edit menu in NLA Editor's header to modify the selected strip in either the Action or Graph Editors: [
Please note that when you add a strip it would be modified from the position it is inserted from in the NLA Editor which means if you add a strip that was originally from frame 1 to 10 and in NLA editor it is from frame 21 to 30 it would be modified from frame 21 to 30 instead of the original 1 to 10. To change this behavior click the little pin icon on the action strip: