Is (was) it an evergreen? It looks very much as though its gone to that great forest in the sky. Try scraping a bit of bark from one or two twigs or branches with a fingernail. If the wood underneath is brown then its dead, but if its still green there is hope.

I can see some other growth around the trunk, the bottom couple of feet - I can't tell if its growth off the tree (certainly some of the leaves look the same orange yellow as the rest of the tree) or whether the green/reddish green leaves I'm seeing are actually a rose or another plant.

Did this tree have flowers at any time, and was it an evergreen tree or one that lost its leaves in winter? Has it been hard pruned during the last few months or year?

Thank you so much for replies. The tree would loose leaves in winter, so not an evergreen. It has never had flowers, leaves were olive green, and a quite shiny. There is a rose just behind the tree which over the years has twined itself into the branches, (not the main trunk though). I have only ever pruned the smaller branches to try to maintain the rounded shape. The main trunk appears solid with green and brown shading. The branches are dry and break easily. I would love to try and save it as love the shape and it fits perfectly in the garden.

It's next to impossible to identify what the tree might be in its current state, and knowing what it is/was might have been an indicator as to what could be wrong with it, but as it stands, unless you can find any soft or weeping spots in the trunk, particularly near or at the base, its hard to know. If you do find any soft or weeping areas in the main trunk, its likely one of the phytopthera infections, maybe at the root, which won't be treatable, or a canker. Alternatively, I suppose its possible that all the new leaves got caught by a sudden hard frost or dip in temperature - if you can't find any evidence of disease on the trunk anywhere, then its a case of wait and see if it recovers or not I'm afraid - its probably worth giving it a good feed with a balanced fertilizer, raked in round the base, in case its fighting off some problem, otherwise, not much you can do.