oops! you forgot the photo!

Thanks Jean - hopefully it is here now.

Japanese maples are prone to an assortment of fungal issues that can cause dieback. Also, containerized maples are more vulnerable to winter cold than their cousins planted inground......although I doubt that's an issue in CA :-)

There is no sure way to determine from the photo what the exact problem is. If there are no viable buds on the naked portion, it is fair to assume that is dead wood and can be removed. Ideally, taking this in to a better nursery (one that specializes in JM's) or to your local extension office for an accurate diagnosis would be most helpful.

I have to tell you I do not hold out good feelings about the damage showing. It looks a lot like a usually fatal ailment called verticillium wilt. While not common in containerized maples, it can happen.

Do whatever is necessary to have the problem diagnosed by professionals. VW can have a long range impact on plant selection and most specifically, on how or where you can add JM's in your garden.

What kind of soil is in that pot and how is the drainage? In addition to what gardengal mentioned, a root problem may be added to the list of suspects.

tj