I'm no expert (I'm pretty darned new actually) - but as nobody has answered, let me try:
How often do you change your water? Do you add anything to your water?
My guess is that your hornwort has used up the nutrients in your water. I have some hornwort and the stuff grows so quick I think it has to be a nutrient sponge!! From what I've read, you'll want to do a large water change (40-50%) once a week, and adding the proper nutrients to your water will help quite a bit.
I'm not adding nutrients yet (that starts this week), but I've noticed that my hornwort starts to look brown around 5 days after my last water change, but after a water change it gets more green.
Any real experts want to back me up? (Or correct me)