My lucky bamboo plant?

Well i got a lucky bamboo plant from my teacher, and she has kept it alive for 6 years so i don't want it to die!! well it is in a short gl vase with pebbles in it. it only has water in it, no dirt. it is in my room on like my tv stand shelf thing and directly across from it is a window and i pull the blinds up during the day so it can get some sunlight, cuz i know u aren't supposed to directly sit it on a window sill. well it was all going good until like a few days ago when the stalk started turning yellow..like any ideas or suggestions to help it? Would greatly appreciate it!! :)