I accidentally over contributed to my 401K - what do I need to do?

I accidentally over contributed to my 401K and I called Vanguard to pull the extra $ out and they said the deadline was April 15th and I'm too late to do this. Is this accurate and what should I do in this case? Should I still request a withdraw for the amount I over contributed by? What happens in this case? Also, should I still pre-preemptively report this on my tax return for 2017? What else do I need to do or know in this situation?