I have a plum tree on my allotment plot. It started to bud in spring. The buds and the branches then died but the trunk of the tree is ok.
Certified GKH Gardening Expert
It could have suffered cold damage, or a fungus or pests could be to blame. You can remove any dead growth and it should regrow. Look for pests and treat the tree with Neem oil (both an insecticide and fungicide) and see if this helps at all.