I just lost my first client have done a massive job for them creating their accounts from scratch from their pile of unmixed mess for whole year, no appreciation.. well it was a horrible disorganized rude client but with such fierce competition in my area it's sad.. :'-(
Happens to everyone at some point (though thankfully still to happen to me but I know will happen at some point as nearing 100 clients now and bound to upset someone at sometime).
You just need to continually market to get new clients to replace lost ones and also try to go beyond the call of duty to try and not lose existing clients.