In many cases what happened was avoidable.  When a client makes a move it is usually for a good reason.  Most of the time it is about how you respond to their needs.

Other reasons which are beyond your control include a fast talking salesman and the more likely financial decision.  While the client appears to be solvent, they may be struggling to make ends meet.  Then you add 10 dollars to something on their bill.  Even though the extra money doesn't really matter, it gets them to thinking, and you are out.

The first thing you do is wish them well and tell them that you will always be there for them.  Many times they are back with you within a month.

The next thing you do is try to fill in your loss.  Diversity is key here.  You need to trigger other income streams to make up for the loss.  Be creative.  It is usually something easy to implement that you were too busy to do before but have time for now.