Go to the bar every night til you find the girl. Ask the barmen/barmaid if she is a regular. Ask anyone who was there with you in your group if they have her number. Dont give up.
This use to happen to me with my Motorola Razor all the time, so I press save, save and done, out of habit! I like the idea of sending a text message almost immediately as a test; next time get a picture with her number and set the contact up immediately.