All perfect praise be to Allah, The Lord of The Worlds, and may His peace and blessings be upon our Prophet Mohammad and upon all his family and companions.
First: that doesn`t invalidate your (Asker) Wudoo`. However, just as you are keen on preserving the validity of your Wudoo`, you should be keen on covering your hair before non-Mahram men since it is incumbent upon a woman to preserve her chastity. Nonetheless, if that man has seen your hair accidentally and there was no negligence on you part, then you aren`t sinful, Insh`Allah, and this incident neither invalidates your Wudoo` nor his since there is no evidence in Sharia which indicates that a woman`s hair is amongst the nullifiers of Wudoo`. And Allah Knows Best.