could be a lot of things. if it happened overnight theyre probably dried out. Are u doing hydro? if so, feel your rockwool cube its probably bone dry. If thats not it then u can put a stake in and tie the stem (loosly) to it to hold it up. Then put a small fan on it (again light is better) and this will induce the stem to go thicker. Post some pics and i can be more specific.