Gosh I know this one is really hard to do. You may have grown to get use to a certain lifestyle, with certain people occupying that space, to finding yourself alone one day wondering where they disappear to.
Be easy to forgive them and do your best to welcome them back when they are ready to return. Understand that people move through life at different pace than you and sometimes it takes them off course for a while, and sometimes permanently. Use this as a lesson to treasure the time that you have them for company and look forward to the day your courses will once again merge.