Certified GKH Gardening Expert
This could be many things. It would be best to know the plant, and the environment, or climate zone that the plant is in. If you can provide any pictures, and detailed information about the environment or any changes to it, then I will be more than happy to help.
Alternatively you can take a sample and pictures to your local extension, and they will be able to help you further. This article will help you find the closest one: https://www.gardeningknowhow.com/extension-search/