I hardly like to tentatively suggest this - but could your neighbour have taken advantage of your absence to spray it?

Well it might recover - if you're in the UK, and while you were away we had that heatwave, that might explain why its done this - its been so cool up till then, that sort of heat would have been an enormous shock, especially if new growth had started post its pruning.

These are, though, shortlived shrubs - average life span is about 10-15 years, though some give up after 8 and others go on longer. If, as suggested by someone else, someone has sprayed the shrub, I'd expect evidence of spray damage on other plants, and if they used something like round-up or glyphosate, or any other freely available weedkiller, that won't kill it anyway, it would just get rid of the foliage for a while. Not to mention that if all the foliage is completely dead on your side, its unlikely to be weedkiller unless the neighbours could access your garden to spray it from that side as well.

Anyway lets really hope that wasn't what happened.

It does sound suspicious to me. I prune a Ceanothus annually in June and the leaves certainly do not die afterwards. It always responds well. I sawed off a few old branches nearly 2" thick to shape it last June and it didn't mind at all. I presume yours looked healthy before you went on holiday.

If so I would suspect foul play in your absence.