I have a conifer—I think it is a creeping juniper—which is only green right at the shoot tips. Everything else is dead and brown. Can I save this 30-year-old tree?
It would have been helpful to see a picture of the plant so that I could form a more accurate opinion. However, I can try to give you as pragmatic an answer as possible: If a plant has reached this age but only its shoot tips are green, then its best days are likely behind it. When pruning the old wood, creeping junipers (you would need to specify exactly what kind) are usually quite shy to produce new foliage. You need to find out why the tips have gone as brown and dry as you say they are.
If you want to try cutting back by around 30–40 centimetres, you should spread approx. 30 grams of fertiliser per square metre around the roots of the juniper towards the middle or end of March. This will encourage new foliage to be produced. As soon as the foliage begins to grow back, cut back the juniper as stated above. If the plant has even just a little green, the likelihood of it producing more foliage will increase. Any young shoots can be considered a "gain", because pruning will encourage these to become bushy again and help the plant to rebuild itself over the coming years.