The most common cause for a laser printer to stop printing red ink is that the printer's toner cartridge has run out of red toner. Issues with the printer's software configuration or mechanical issues with the printing mechanism also cause laser printers to stop printing red ink.
Color laser printers use separate black, cyan, magenta and yellow toners. Since a separate toner is used for each color, a laser printer can run out of a specific color before the other types of toner, especially if one color is used more often than the others in printed documents. It is sometimes possible to replace the colored toners individually to resolve this issue, but this depends on the design of the printer. Some color laser printers require replacement of a single toner cartridge for all colors at the same time.
Issues with the printer's software driver sometimes causes problems with color printing, so examining the settings of the driver is a good first step if the printer's toner levels are adequate but the printer still fails to print correctly. Also, while most laser printers use a single mechanism to print colors and blacks on a page, some designs use a separate printing mechanism for each type of toner. With these printers, a mechanical failure in the magenta printing mechanism can lead to red colors printing incorrectly even when other colors print without issues.