That's a hard one to answer, actually. You need to give the reader the sense that spending their time on your story was worthwhile, but that doesn't mean everything must be resolved.

There is a term called the Story Question. "Will the hero defeat the mastermind, diffuse the bomb and save the orphanage in time?" If you left any part of that question unresolved, it wouldn't be acceptable, because you've already made your readers need to know. But, less critical questions, longer viewed questions, that's the sort of thing you can leave open for interpretation or unresolved
Say, the hero was sent back from the future because the mastermind knows that one of the orphans may grow up to thwart his plans for world domination. It's okay to not show which kid grows up and fulfills the prophesy. Or even if the prophecy is correct. An orphanage full of still-breathing kids is plenty satisfactory enough.

This is why there isn't a pat answer to this. You need to resolve the story question, but first you will have to determine which conflicts are covered by the question and which aren't. Those that aren't resolved can become part of the story question of a following book.
...
Regards,
Eric Fretheim
Assistant Prep Leader, 2015 October NaNoWriMo Prep Challenge

"It is perfectly okay to write garbage-- as long as you edit brilliantly." ~C.J. Cherryh

“No, writing 50,000 words in a month is normal. You are not crazy. This is not insane.” ~Teri Brown