That happened with one of the leaves on my plants, but it didn't spread anywhere else. I looked it up in one of my books, but I couldn't find anything with just a completely yellow leaf. But what it usually means is that there is some sort of deficiency in one of the mobile nutrients (nitrogen, phosphorus, potassium magnesium or zinc). When there is a deficiency of a mobile nutrients, the plant will take the nutrients that are stored in the first/bottom leaves and transfer them to the most recent growth to try and ensure itself continued growth. So I can't say which it might be because there isn't a picture that I can compare it to, but if you're not already, try starting to use some nutrients. I'm using FoxFarm now, and it calls for GrowBig and BigBloom in the first few weeks of seedlings and then just GrowBig during the vegetative growth. If you go to a Grow Shop to get some, ask for the FoxFarm feeding schedule and it will tell you exactly how many nutrients you'll need and when.