You didn’t give much information! Could be caused by many things, such as a harsh winter, drying winds, poor soil fertility, insect damage, diseases, etc. You really need to know what kind of hedge it is, then you can try to find out what the problem is. The first thing I would check on is if they are getting enough (or too much) water.
If you can’t figure out what kind of tree or shrub your hedge is, then I would suggest contacting an arborist to come out and look at it: http://www.asm-isa.org/arboristlist.html
Also please check out the following website: http://www.naturalwaylawn.com/tree/
Hope this was helpful.