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Send her a text telling her you're really sorry etc...

No it's not... just ask her to reschedule, and let her choose the place this time. Laugh about what happened with her (with self-irony, NOT self-deprecation), it's actually a good bonding experience...

Just tell her something like "I'm sorry you're feeling blue... how about I take you to dinner tomorrow, to try and cheer you up :-)? I promise I'll do my best ;-)" or something silly/fun like that I...