If the sandy base sits on top of heavy clay soil, you may have a perched water table. Water doesn't readily move from soil of one type of texture into another, so although the soil near the surface may be sandy, water could be collecting just above the clay subsoil, and sitting in the root zone, causing root rot. It's also possible that the thyme developed a foliar fungal disease. If conditions are humid and wet, it can develop fungal leaf spots.
If the foliage is entirely brown, it would be helpful to cut back the plant and destroy the dead sections to prevent fungal spores from overwintering on the dead tissue.