If you have her email, write her a note... Or SMS her... Just ask "Not sure what happened. You interested in trying to meet again?" If you have neither, then hopefully you'll get her machine/voicemail. Something short, to the point, and non-accusatory without being smarmy. Or you could go with the "oops, I got the wrong number... Who is this? Oh, hi! ...." routine. It could be something came up, or she flat out forgot, and was too embarrased, shy, or convinced you'd blow her off if she contacted you. It is a bit late to call, but one call a stalker does not make.