It sounds like you may have bashed your tibial tuberosity - I did the same thing about eight months ago (alcohol and stone stairs, sigh) and it's still bl**dy sore to touch. If this is the case, you've probably bruised the bone which can be very painful and take a long time to heal - you may also find when it does heal that there's a slight dent or lump in the bone at the point of impact.
However, it's also possible that you've damaged the patella tendon; you don't mention any pain when you tighten your quads, but if you do experience any pain when walking up stairs, for example, or when you pull your toes as far up as possible, it might be worth seeing a musculoskeletal physiotherapist just to be sure.
If you have damaged the tendon I doubt it'll be severe (you'd know about it if you'd ruptured or badly torn it); probably a minor tear or even just a bit of scar tissue from where you mashed it. A good physio should be able to sort that out within four to eight sessions, although I'll warn you upfront that it'll hurt - if it doesn't, find another physio.
Best of luck; it's probably nothing, but if you're worried have it checked just to put your mind at ease.