If it was a straight thwack to the knee and there was no twisting or torquing of the knee involved, it's likely just a contusion that will hurt a lot but will not have an effect on the structural integrity of the joint. That is, you should be able to go hard with the rest of them and you may even be able to keep up riding!!

The only thing that I would be mildly concerned about is your iliotibial bursa. This is a tissue under the distal part of the iliotibial band around where it joins into the knee. If you hit this directly, it may swell quite a bit and interfere with how the soft tissues shift and glide around your knee while you're cranking it out. If this is the case, it would be quite painful to ride and will get worse the longer you ride. If this happens, you should cut out the riding. Bursas are tricky things to settle once they're angry.

If it's a bruise, it might be somewhat sore, but should be tolerable and shouldn't get worse as you ride. So, try to be aware of the difference and monitor your activity accordingly. And yes ICE is nice. I love ice, especially in a good margarita (on the rocks, not blended).