crying?As a sentence-ending phrase, の makes a question sound explanatory or seeking confirmation. The verb changes form as needed, as in 泣いたの? (Did you cry?) and 泣いてるの? (Are you crying?).
cryingの nominalizes 泣く, turning “to cry” into “crying.” In expressions such as 泣くのか and 泣くのかな, it can instead be part of an explanatory question meaning “will you/he cry?”