The first thing anyone is going to need to help you is a picture if at all possible. I am still new to this game so I wouldnt be of too much help, but if you could put a description as far as medium, nutes, veg or flowering, ph, watering and how often, room temps, lighting, how far away lighting is the wattage and everything, this should help others help diagnose your issues. Then, after you get an idea of what the issue might be, I suggest reading and looking into how these things happen, how to prevent, and how to fix. Its a lot better for you to understand really understand these issues than to just say, ok, they told me it was this, ill fix it. If you actually understand the hows and whys of it, it will help prevent it from happening again.
GOOD LUCK