From the photo, I can see evidence of dieback in the top of the tree, but cannot tell what the problem may be.  Often time symptoms such as dieback like this in the top of the tree can be related to problems on the lower part of the tree.  Problems could be insects or disease such as aphids, borers, scale, canker, leaf spots.  Other problems could be girdling roots, planting too deep, over-mulching, mechanical damage from weed trimmers or lawn mowers. Ornamental plums are considered to be relatively short-lived trees (20 years), so your tree just may be towards the end of its life. 

Before you spend money to hire an arborist, try taking photos and a sample to your local Extension Office for diagnosis.  Find your local office here:  http://www.ext.vt.edu/offices/index.html.  If you decide to hire an arborist, you can find one here:  http://www.treesaregood.com/findanarborist/arboristsearch.aspx.