Looks like Leaf Spot, and a severe lack of nutrients (and possibly water, too).

If it's in a pot, get it into a larger one with fresh, good quality compost, or (preferably) get it into the ground.

Frost or wind damage or both, most likely frost. Cut it right back to healthy growth and it'll regrow. The yellow leaved form is a bit more sensitive to frost, and if it had new growth which got caught, that would explain this. Should be planted in a sheltered, sunny spot and preferably not in a frost pocket, but this happens anyway some years. Can't tell if its in a pot or in the ground, so can't advise about that... there may also be some slug/snail damage, so use pellets.

Could be spider mites if that is webbing that I see at the top of the plant.

We don't get those outdoors over here, Loosestrife2 - might be totrix moth caterpillars if there is webbing, but most insects really hate the taste of Choisya. I don't blame 'em, I hate the smell of the leaves if they're bruised...

I agree with Bamboo; frost and wind damage. Does need a sunny, protected spot.

I had a yellow Choisya in my last garden and it often got hit by frost. Three consecutive hard winters killed it off. As Bamboo says, cut it back and it should regrow.