If the phones display has stopped working correctly, you should try restarting the phone. Sometimes, that is all it needs.
If that doesn't work, try checking the settings for the display. Occasionally apps, as well as updates, can change the settings.
Also make sure your phone is fully updated. Missing updates can leave you with problems as well.
Next, should none of the above help, would be to try a "Factory Reset", which will cause the loss of 'some' of your personal data, and should that not work, a "Hard Reset" which would remove all your personal data. So I would save those as a last resort.
If nothing above works, then you would have to replace the display, or worst case scenario the phone.