Who is responsible for a fence repair between neighbors ifthe wind blew it down?

Whose fence is it? That is, on whose land does it lie and who therefore owns it. That is the person who is responsible for fixing it, if she or he chooses--a person is under no obligation to have a fence at all, or put up one that fell down, unless and only to the extent there is some legal document (e.g. HOA rules) specifically requiring it. So the most important thing is to determine whose fence it is in the first place, since that determines who owns it and therefore who has both control and responsibility for it.

That said, if one person really wants the fence and the other one doesn't, probably the person who wants the fence should pay to restore--even if it's on the other person's land, they can work out an agreement by which this is done--or else put up a fence on his or her own land. Think of it this way: Suppose the fence is yours (on your property): if you want to restore it, you can; if you don't want to, you don't have to--but your neighbor could pay you to make it worth your while, or could up his own fence. If you turn it around, it works the same way in reverse if the fence is on your neighbor's land.