Hi:
This is Kimberly from landscape-solutions-for-you. Thank you for taking the time to ask a question on your Weeping Cherry tree. It sounds like your tree may have some type of disease, fungus, or insect infestation.
The first thing you should do is take your fingernail or a small sharp knife and scratch the surface of a branch. If it is green, the tree is still alive. If it is brown, the tree is in stress. You can check the leaves underneath for any form of fungus or white. If there is, I would recommend taking a leaf or two to a local nursery and see if they can identify the problem.
You can also angle cut a branch and take it to a local nursery. If your branches are becomming brittle, it could be a sign of your tree not getting enough nutrients from the soil. You can have your soil tested, or purchase a good quality fertilizer and loosen the dirt and add a hand full of the fertilizer. Make sure the tree gets plenty of water. I hope some of this will help you with your Weeping Cherry. Let us know how you made out and the best to you. Thank you again for visiting lanscape-solutions-for-you. Feel free any time to ask a question or help some one else out. Have a great day!