Hi there, thanks for your question. If your printer is an HP OfficeJet 8600 plus, that would be an inkjet printer so it would likely be an issue with the print heads being covered in dry ink or blocked. If that isn't your printer and you do have a laser printer (that takes toner rather than ink), there could be a number of reasons depending on the printer model, but it would most likely be a problem with a drum or fuser.
Have a look at the following page, which will give you tips for both inkjet and laser printers:
What causes faded prints and how to fix it. If you'd like any more help, please let us know and we'll be happy to give you more advice.