If your using flourescent get the light within 1 inch of the top of plants, if using other lights put your hand at top of plant, if you can feel the stinging of the heat move it back.
If you just have it in the windowsill, you need to get it to a place of more light, pronto.
You can still save it, but you dont want it to get any stringier.
Take some dirt and fill in to the bottom of the leaves.