I agree with Welshonion, I'm afraid. Forsythia is prone to a certain amount of die-back in my experience. And, yes, the rule of thumb of pruning about one third of a shrub each year is best but you probably gave the poor old thing one helluva shock!
Best course of action is to know when to admit defeat - and some plants reach the end of their natural life, and need to be culled anyway.
Ask yourself if you really, really want forsythia. It is a wonderfully bright harbinger of spring. And dull, dull, dull the rest of the year, imo. I didn't weep any tears when an inherited one turned up its toes on me!