Alert your rep immediately. The emergency contact details will be in your welcome packs text to you on your arrival day and on your wristbands that you receive when you check in.
We will then organise a search team and notify the authorities, we will keep you up to date with our progress. It is important not to panic - in 99% of cases it is simply a case of your friend going home with others, still being out or having fallen asleep in a silly place.