One of the main causes of stem collapse is damage from wind or slugs. If you inspect the withering stem you can often see the damaged point, which sometimes is traceable to the point where you secured the stem to a trellis or structure without enough flexibility to allow it to handle the wind.
If you look at the stem on the lowest part of the plant you may see the outer ‘bark’ stripped away. This indicates that slugs may have damaged the stem to the extent it could no longer function to support growth and it will wither away.
For either of the above issues (on a larger clematis), starting at the top, remove the stem in pieces, working your way down and cutting and removing sections. When you get to the bottom, cut the stem at or just slightly below the soil level and discard the stem vs. compost it. Usually the plant will replace the stem immediately with a fresh shoot; sometimes it does take a few weeks for the growth to initiate. If the clematis is containered and there no other stems, or few stems, monitor and do not over water in this stage while there is little to no vegetative growth.
Fungal issues are rare, and as plants build resistance to your garden ecology, there should be few issues. Consulting an expert at your garden store is always an option. Often rose formulas for a variety of issues work well, but always test them on one clematis before using them on all of your plants.