Sure, I can give you a guess and a vague tip without seeing a photo. 'Cause I’m 98% sure I know what’s going on, just from your description.
The hedge has been trimmed improperly. Probably over a long period of time.
The problem is probably twofold.
Overall shape. See, I’m guessing that the hedge is supposed to be in the form of a “wall.” This is a common style, but most people make a big mistake with it. There is a natural tendency to trim the hedge more tightly towards the ground, and leave it a bit wider towards the top–for a modified V shape. This is incorrect. If it is in a V-shape, the lower part of the hedge will get less sunlight and become more leggy. The top part will become bushier and denser and block even more sunlight. The hedge should be trimmed in more of an A shape. This can be done subtly, so that the hedge maintains its vertical look.
Surface trimming only. The hedge was probably trimmed with an electric hedge clippers. If someone just keeps trimming off the outer part of the hedge without any attention to the structure, the result will be a leggy and ugly hedge. If the “growth tip” of each little twig is cut off, the plant will usually create two new growth tips. But the new growth will often be very close to the end of the branch. So, the hedge will get really bushy at the very outside, but the sunlight will be blocked to the inner part of the bush. The remedy to this is to selectively prune out branches in areas that are getting a bit tangled.
I’d guess that your hedge is far from dead. But it needs some rehab. Bad news–it will take a couple of years before it looks really good. Good news–it won’t take a lot of work at all to rehab it. (Don’t get tempted to rip it out and put in a new hedge. It will be a lot easier, cheaper, and more effective to just rehab this one.) The rehab will consist of reshaping the hedge, pruning out some of the branches, and enriching the soil to help the hedge recover from the heavy pruning and push new growth.
I’d suggest you purchase a good, comprehensive book on gardening and landscaping. (Rodale’s big Organic Gardening book is a good one.) The diagrams will show you how to prune and trim the hedge, and if you have a house, you should have a good gardening reference.
And of course, you should figure out what exactly you have. You can take a branch down to your local garden center and ask them.
In the meantime, if you get tempted to trim it, just remember that it should be in an A-shape, not in a V-shape.