If the tendon feels like its slipped out, put it back in place (by extending the leg joint and pushing gently on the knee with your thumb). Tape the joint to keep the tendon/knee cap in place and put it in a chick chair so it doesn't have to put weight on the leg. Hopefully the chick will recover in a few days, It may just need to form the knee further to accommodate the joint better. If it doesn't fix itself in a few days, it may never. Sorry.