The curtain stopper, which the curtain hook attached to, had broken off. I checked MakerWorld, but I couldn’t find a model compatible with my rail system. So I designed one myself instead, and it works perfectly fine. The hook has a threaded section that screws into a slider inside the rail. You rotate the hook to tighten until it clamps into place, after which you hook the end hook of the curtain into the eyelet of the stopper.