Consider the season if your tree has dropped leaves. It's perfectly natural for deciduous weeping willows to drop leaves from fall to spring.
On the other hand, weeping willows tend to lose their leaves in summer during periods of drought and must be irrigated immediately to prevent death. Jun 08, I have a willow tree in my front garden that is about 9 years old and has always been in full leaf at this time of year but all of a sudden it is losing leaves rapidly and I can't find any obvious explanation. There is no apparent fungus or insect and the tree is.