It does sound as though your shrub has come to the end of its useful life, jeannie99 for whatever reason. They don't tend to suffer from pests, but maybe root rot? I suspect that when you dig it up you will find the cause at the roots. With the very cold winter, which can affect even the toughest plants, and the very wet July and August we have had it is probably just that the poor old plant couldn't cope. Is it it a sunny, sheltered spot with room around it to allow for air circulation?
I would dig it up, thoroughly dig the area and give it some tlc with maybe some grit to aid drainage and compost/manure to feed the soil and then plant a new choisya.
Fingers crossed that will solve the problem...choisyas are beautiful plants, aren't they?!