We all make mistakes, so don’t feel ashamed of yourself. The sun may have burnt some of the LCD, so you will have to replace the LCD. Turning the device off probably won’t help, but it may go away over time, it depends on how long it was in the sun, and how hot it got. There is a repair guide on iFixit that can show you how to replace the screen. If you don’t want to replace the screen yourself, you can take it to a local repair shop.