Adding a picture of these grey lines to your question would help us understand the problem better.
However, issues on the LCD are usually caused by a bad LCD. At any rate, troubleshooting iPads (and iPhones) requires access to known-good parts and the first step would be to open up the device, inspect connections and look for obvious signs of physical or liquid damage. Once that is ruled out, then you have to replace the LCD to see if that solves the problem or not. If it doesn't, then the problem would be logic board related which would require a micro-soldering solution.
All of this is still less expensive than replacing the unit though.