If you can get to the menu, see if you can go the picture settings and adjust the brightness and contrast settings. You can also try unplugging the TV for 30 to 60 seconds and see if it helps. If you have something connected via HDMI cable, you can try a different cable. You could also try changing the mode with the * button on remote such vivid, movie. You can try a system restart too.
If those things do not work, it is possible that the TV may be defective, or the screen has some issue. If it is new, I would suggest returning it to where you bought it.
Otherwise, you will have to contact the manufacturer TCL's, customer service directly. Roku may be able to give you some ideas to try, but they do not provide technical support for third party manufacturers.
