I have two printers that have stopped accepting jobs via their USB subsystem. CUPS sees the printer USB ports, has been able to send jobs to them in the past, but can't talk to them:
cas@odysseus:to-print> lpstat -a | tail -4 samsung-ml2010 not accepting print requests since Sat 04 Oct 2014 07:09:05 AM CEST - reason unknown sharp-al1035 not accepting print requests since Mon 06 Oct 2014 11:14:09 AM CEST - reason unknown
Calling Samsung's hotline first gave me a reference to a "service partner" who says they have stopped being a service partner, too much trouble for the income associated with USB subsystem problems. Second call gives a service partner who says it is cheaper to buy a new printer than fix a USB subsystem problem. I have not tried to talk to Sharp yet.
Probably there each reason has its own particular reason for why it won't talk CUPS and the printers cannot be made to work via USB. Can any further information about the problem be divulged using the CUPS side, though?