Hope the b/m contexts would help a bit to clarify, feel free to ask...
Situation 1: ´Have you already talked to her about it? Why are you just sitting there and not doing anything about it?´ = 'There is nothing else to do ...as I screwed it up I am afraid she will not accept my aplogiesapologies anymore'. = It indicates he only cries over spilt milk but did not make anything to change the situation (maybe the situation will change if you add 'for now'after 'There is nothing else to do' and he will take the courage to change it, depends on the following context).
Situation 2: An employee has just completed all the tasks his boss gave him, and his colleague is asking him (as he was pretty fast in completing them even before deadline) :
'Why are you having such a long break, not doing anything and still in the office?' 'I have completed all the task our boss gave me and I have nothing to do for now (there is no other task left his boss assigned him).