One of my 2 year old clematis flowered for the first time a few weeks ago . The blooms lasted 2/3 weeks and then the plant started to darken and brown rapidly and now appears to be dead . The other plant appears to be very healthy , they are both in the same bed against a wall and have both been treated the same .
Watering issues is the most likely issue. Make sure the soil is moist but not soggy; and that the soil is well draining.
Excess moisture could lead to root rot or even fungus issues with the vine.