Might want to think back to how you prepared the seed - you cleaned off the outside fleshy part and revealed the harder inner pit. Now at this point did you break open the pit to reveal the fleshy inside part and plant that, or did you leave it in the pit to break open naturally? We can tell the root came out, good, and the shoot appeared, okay. Attached to the shoot will have been two round leaves, (cotyledons), as you say, about size of a 5p. The thing knocked off might have been part of the outer shell if you did not remove it, or it might have been one of those cotyledons. Since you don't have any cotyledons left (from the image) this is not good news. The cotyledons carry with them the growing point and if when you lost the cotyledons you also lost the growing point then there is nothing further to expect from this experiment unless you are very lucky.
Before you give up, tease away the soil from around the stem until you get back to the point where it becomes the root and not the stem. You may just see a new growing point if the cotyledons stayed submerged. If you find signs of growth down there then give it light and positive thoughts.
It looks like 95% says you have to start again. It is worth the effort to break open the pit, but do it very carefully to avoid damage to your hands and the embryo shoot.