You could cut all branches and trunk very hard and it should respond by budding back on old wood and eventually grow new branches

Mike's recommendation above is really the only way to go. You would just have a hot mess if you just cut back hard as Todd suggested - no JM should ever be pruned in that manner ( I'd rip it out first!).

Tip die back on JM's is pretty common and the harder the winter, the greater the die back usually is. Typically, you wait until the tree is completely leafed out and remove any dead wood. It will be obvious - dead wood will appear as pale gray whereas viable wood will be dark in color. It is actually a suggested practice to cleanly remove dead wood frequently, as it more easily harbors disease and insect issues which you want to avoid with these trees.

Make sure the tree is well watered this summer - JM's are not at all drought tolerant and will need periodic deep waterings during a dry summer. If this is something that cannot be managed due to water restrictions, etc., then I'd consider removing and replacing with something more suited to a drier environment.

I am devastated!! Just got my arborist to come out and spray and it turns out they tree has a root problem. He said the roots are girdling and suffocating it :( Only has 10% chance of survival.

I feel your pain, I Lost one of my gorgeous pom pom trees this winter I was floored when I looked out and it was turned over, really odd high winds that night, must of had root rot or something...even tho it broke clean at the bottom....sorry :( I am anal w my bushes and trees

Just wanted to update!!! My tree came back this year after cutting the girdled roots!!! So happy!

Nice..now get rid of that dead wood ..