Sedums are deciduous herbaceous perennials and will die back naturally in late autumn, when the first frosts blacken the stems and foliage. It could be that an early frost has caught the plant if it is in an exposed location. You just need to cut back the stems to ground level and new buds will emerge next year. Also take a look at the crown of the plant. If it is wet and slimy then the plant may be suffering from crown rot. This is a common problem in sedums and other succulents which occurs when the plant receives too much water, or when the water supply is too sporadic. Gardening on heavy, poorly drained soil also increases the risk. If the crown feels squidgy then the damage is already done, and it is best to dig the sedum up and compost it. Next time, before planting a sedum try adding plenty of grit to the soil to improve the drainage.