My avocado seed was growing fairly well, good size root and was staring to grow a stem, until I knocked it over and it has slpit in two, one have got the root, the other half got the stem, will it still grow? How can I hope to fix this?
Likely yes, just put the two pieces back together they way they were and this time plant up to the neck in potting soil, not water, leaving the top of the main shoot just visible. Keep the soil moist and well drained. Even though you have lost the main tap root it may generate another, depending on what cells are left with the main shoot. This is quite a bad shock for the seedling, so it will take some time to recover. If you don't see a sign of the shoot appearing in 6 weeks or so, or if the tip of the main shoot looks brown, black or just disappears then remove it and check the inside again.