I think it depends as much as on whether Plan A has been a Regular or not, tcm.
If she hasn't been, I'd be inclined to start planning now; then send another text telling her that if she isn't going to reply, she's leaving you no option than to look elsewhere.
If she has and you want to keep her as a Regular, I'd begin planning; but send her a different text, maybe asking if everything's okay with her and is she still okay for tomorrow. If no reply within, say, an hour, put Plan B into practice.