Hi, Depeending on the thickness of the lines matters. If the lines are Black and about 1-2 inches thick it may be a bad bottom buffer board or a bad panel (screen).
If the lines are thin 1/16 to 1/8 inch it is most likely a bad panel.
You can open up the set and depending on the model, see the thin board/boards running across the bottom with ribbon cables going to the panel. You may be able to attempt reseating the ribbon cables, but be careful as some retainer plugs break very easy.
If the lines are thick black and the reseating doesn't correct it you may be able to buy the coresponding buffer. Unfortunately some bottom buffers are actually part of the panel. If you are not comfortable taking it apart then call a professional but be aware labor charges start around $200 not counting parts, panels start at $600 up to over $1000
hope this helps
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