My daughter and I have built a hydroponics system for a school project. It has been running for about 2 weeks and the plant are growing well until yesterday. I noticed the roots are starting to turn a greyish/white color and the plants starting to droop. Can anyone tell us what is causing this and how to correct it. Thanks

Drooping plants indicate exosmosis ( Water being sucked out of the plant)

That surely means water concetration or solution concentration is greater than the plants can accept .

You have to change water / solution and put new one in the tank

Sounds like some kind of nutrient imbalance or pollution in your water. I'd change it and hope for the best.