Dead hedge?!?

Can anyone help me work out what has happened to our hedge. We have two like this separating our garden from the others in the terrace row. One is still looking okish but the other one, in this pic has lost its colour and died over the last few years. The pic shows our south facing side of the hedge. The north facing side in the neighbours garden still looks predominantly green. We can't lose these hedge as they provide privacy and shelter in our wee terraced house garden. Can anyone help? What do you think the problem is? Can we do anything to save it? Or could you suggest something that would grow up and cover it mostly, without ruining the neighbours side or killing the whole thing!? We live in east central Scotland and the side of the hedge is facing south. It's not the only  hedge in the row of house that might be effected with this. thanks in advance of your green fingery wisdom ????????