HP LaserJet ink doesn't adhear properly, why?

I have an HP LaserJet 4 that I LOVE. I had put it away for about a year and just resurrected it. The printout looked as thought the toner was out... uneven printing with toner on both front and back (like when it is running out of ink). I have since baught a new cartridge and still all grey print results - as if my doc was a grey doc. Can it be the heater for the toner is out? It seems the toner is not adhearing properly. HELP!?