Thank you for your question and I understand your concern.
If you are able to manually spin the head and it's not locked up, the drive shaft may not be slid up into the clutch cover and clutch in order to spin the drive cable.
If it's a new and covered under warranty, I recommend taking it back to the store for repair or a new replacement.
If you get to the point where you need to have a service technician fix this problem, you can carry the Weedwacker into your nearest Sears Repair Center or Sears Store. Here is a link for the Sears Service website that has information on the nearest repair facilities in your area:
Sears Home Services