I have a Box Topiary that is starting to go yellow ?

Excess sun can cause leaf discolouration.Boxus is susceptible to blight. This sounds like poor drainage and water logging which can cause roots turning blackish and brittle trim damaged roots and trim bush also, then repot (as below). If its limited to leaves generally turning yellow repot in J I no3 with good drainage crocks in bottom of pot, stand pot on feet to aid drainage. Boxus hates to be dry so fingers crossed it wasnt too bad.

Hi Doakes and welcome repot in fresh compost the stagnant water will have stopped any oxygen getting to the roots and without this the shrub can't survive. When you repot remove all the existing compost/soil and then repot in clean compost/soil. Water to settle in and keep and eye on... may still die but it is worth a try.