actually i dont know your phone, but you can try clicking and holding the power button to 10sec or till it shuts down. You can take the battery out. Or just wait till the battery is empty.
Hi i once bought an iphone 4 screen which worked fine first and after a wekk i could see some lines on the screen. Its a manufacture problem. Go to the guy who repaired it and let him switch the screen. Yes its a cheap screen and something like this could happen but he definetely sold you a broken screen.
actually yes, you ca replace it but it also could be a software problem try to restart the device. If its not working you can also try to reset everyhting (factory reset). if this all is not helping, then its a hardware damage and you need to replace it
