I had two client computers over the course of a week have two copiers disappear from their list of printers. All of other printers remained. And when I tried to re-add the printers - whether via the printer server or as local IP printers, it would fail. It was isolated to my two Xerox printers.
I don't remember where I found it, but I found a reference to a problem with the print processor. The print processor is isolated to each model of printer. In the end, I have to delete the print processor data from the registry for Xerox printers and then reboot. Problem resolved. Of note - these machines were each infected with malware the week prior. Several days passed between the infection and the issue - so I can't draw a correlation, but just to note.
Here's the registry entry I had to delete: