Bummer chinacat. Sorry to hear that. It happened to me with a Sweet Tooth mom that had a rockwool plug around the stem for too long. Drat!

Here's a hint about adding soil over the stem:
You CAN do it, but never on from-clone plants, and ONLY up to the node where the cotyledons had originally been, and then, only before the stem grows that dry, flaky bark that mature cannabis plants get. The moral of this story is, give ample light early to control stretch so you don't have to think about this!