I would start off by checking if the warranty would cover this first. If not then check to see if the cable for the screen is seated correctly on both the motherboard and the lcd.
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=== Update (01/04/2019) ===
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@sylviavila Since you have checked the cables and they are seated properly, and the external monitor does work I would connect an external monitor up and run the diagnostics on it first Page 27 of this [https://download.lenovo.com/pccbbs/mobiles_pdf/tp-e480-e485-r480-hmm-en.pdf|LINK] Tells you how to to run the tests. If everything checks out but the laptop screen still has issues then yes my next step would be to replace the screen. If after changing the screen and it still does the same thing then you have an issue on the Mother Board somewhere.
I would start off by checking if the warranty would cover this first. If not then check to see if the cable for the screen is seated correctly on both the motherboard and the lcd.