Yikes, Tara I can see how you'd be worried. I have not that much experience either, but here's my take on it. As long as your plant has healthy looking leaves and some healthy green roots with growing tips - I can see a great big one in your picture - then I don't think it's in trouble. Your plant has some older, brown roots (mine have those too) and maybe these roots have just reached the end of their useful life? Or maybe they have a case of new algae growing on them - some of mine have a lot of green algae growing on them, it doesn't seem to hurt anything.
For this to happen so suddenly is very odd and would indicate to me it was something in the water or something that got on the roots from somewhere else. Most diseases or bugs would not happen suddenly like that.
Can you get a magnifying glass and look closely at the yellow roots to make sure it's not bugs or mites or something? If they go white or grey again when they dry out, I'd just keep an eye on it but not worry as long as the plant's top growth isn't affected.
