The best time is late fall or early winter, Sheila, when the plant is dormant. It's important to get as much of the root ball as possible, and, since hydrangeas have a fibrous root system that holds soil, moving a mature hydrangea is going to be a very weighty proposition. Keeping the plant well-watered for the first year after transplanting is crucial. If you don't have a good location for the hydrangea at your new home (morning sun and afternoon shade is ideal), leave it where it has been happy.
Note that the objective is to maintain an optimal amount of roots (as mention by Douglas) to ensure good survival.
Get yourself one of those tarps woven polyethylene, very durable, easily found in DIY store.
Pass the ball under the tarp, move closer to the ball as much as possible, and then tie it around the shrub.The cover will prevent the roots being damaged too much during the transport.
I have a hugh lavander plant that is totally brown is there any hope or should I cut it back?
and start over with a new plant?
@Sheila R You had a terrible winter. Cut it back and wait to see if there are signs of new growth.