Why have my buxus plants are turned yellow? How can I bring them back to green?

Are they growing in pots? Have you been feeding them, and what with? If they are in pots, have you been watering them and are there drainage holes in the pots?

yes they're in pots and we have drainage holes in them. But I'm not feeding them, can you advise me on what to feed them.
sorry not very bright when it comes to gardening!!

Depending on how long they've been in pots, that might be the problem, lack of feed. It's too late in the year to feed them now, though you could stir in a bit of fish, blood and bone into the top of the compost. It could also be drought, though, if you've not kept up with watering the pots, specially if they're potbound. Pots need watering even when it rains because the rain doesn't get into the pot. If they've been in the same pots for more than 2 years, they may be potbound and need larger containers, so check the rootballs by turning them out and seeing how dense the roots are. You can repot them this time of year if necessary. Next year, feed monthly with a nitrogen rich feed like Phostrogen or similar between March and mid August.

Don't think they're pot bound - only potted them in the summer. I'm good at the watering but failed miserably with regards to "feeding" them.
Thanks a million for your help and time.