Assuming its happened recently and not in spring, any chance of a photo of the caterpillar?

It's likely to be viburnum beetle. You will need to hard prune your bush and burn all the branches that you take off - the eggs of the next generation will be destroyed. It will take a couple of years to recover from this. Unfortunately the beetle larvae will probably be back at some time in the future.

I don't think it will be Viburnum Beetle Landgirl, if the leaves are eaten within an hour at this time of year - the larvae or caterpillar stage is well over by now, they're in the ground pupating for next year by end of June mostly. Any damage done now would be by the adult beetles themselves, but they'd not defoliate like this in such a short space of time, and Duchessk describes caterpillars, not beetles.... Unless, of course, the description and the problem occurred back in May/early June, when it likely would be V. beetle larvae....