Half of my wisteria is dying/dead

Hi, my wisteria has bloomed for the last 18 years, but this year, half of it didn't bloom and now looks dead with twigs that snap.  I really can't figure out what's happened - there is no fungus that I can see.  Would really appreciate any advice - can I save it?  As parts of it are dried twigs, does that mean these are too late to save and should be cut off?  How far do I cut it - right back to the stem?  I am pretty upset about it, as every year it has been absolutely magnificent.
thanks