Email to HR (Vacation denied)

Hello,
I hope I can get some help here...

I am currently on mat leave and sent my vacation request to my supervisor cc: manager. Later, I've been informed by my supervisor that the unit director has denied my vacation request due to operational demands, and need me to return to work on time in early this September.

My situation is, I was planned to take three weeks vacation prior to my mat leave start, but then my child born three weeks early. Therefore, my mat leave kicked in due my company's policy, and so my three weeks vacation then carry over to this year (but now I didn't get granted).

My supervisor suggested me to email HR and ask them what would happen to my vacation, because now is not possible to take them...and I don't want to lose them, is there any options?

Thanks everyone.