Hillary Clinton won a lot more more votes than Donald Trump, almost three million, but low turnout in key states wasn’t what defeated her. A lot more people voted in 2016 than in 2012, about seven million more to be precise, or five percent more. Only in Pennsylvania was turnout lower than in 2012, among swing states. See 2016 popular vote tracker at Cook Political Report.
Early reports of low voting turnout turned out to be wrong. It took a long time to count the absentee votes in many states.