It's hard to say from the photos, as there aren't any distinctive symptoms or indications of a specific problem, but it might be root rot (does the roof downspout outlet empty nearby, which may have saturated the soil?) or an insect borer lower in the stem, cutting off the moisture supply to the stem tip and causing wilting. Even if soil wetness didn't impact other Dahlias in the area right away, a plant with a presumably weaker or smaller root system (which likely explains why it was stunted or behind on growth for so long) would be affected by root stress to a greater extent than a more robust plant.

If the soil isn't too dry (despite the rain, it might be if there's enough rain-blocking overhang or wall "rain shadow") and the plant doesn't need water, then there's nothing that can be done except to wait to see if it recovers. If root rot is present, it's not treatable, and overwintering that particular tuber might not be successful.

Miri