horizontal lines on the display screen for the first 20 minutes after

“early 2013 Mac book pro had some rain water fall on it. Since then there appears horizontal lines for 20 minutes until the computer heats up. So, for twenty minutes I can not use it as the track pad does not function. After 20 minutes,then there is no problem. How to fix it. If an external monitor is connected to the computer, there are no lines on the external monitor.”