Firstly, stay calm.  You probably aren’t very far away and it won’t take long for the tour leader to realise you are missing.  Use your cell or Blackberry to contact the travel agent, or if there is no coverage where you are, approach a hotel or safe looking place of business and ask to use their phone to contact the tour group.  In any case, your tour leader will undoubtedly give everyone clear instructions on what to do in case of separation and it is very unlikely that it will happen.