Hi,
Have you checked that the [https://pcsupport.lenovo.com/au/en/products/laptops-and-netbooks/legion-series/legion-y540-15irh/downloads/driver-list/component?name=Display%20and%20Video%20Graphics|latest video drivers] are installed?
''If you cannot'' get Device Manager to show that the Nvidia adapter is installed OK by uninstalling/reinstalling drivers etc, then it may be a GPU problem on the motherboard and possibly also a display problem as well
''If you can'' get Device Manager to show that the video adapters are both installed OK and the display still has lines then try connecting an external monitor to the laptop and check if its display is OK. Check using both adapters, Intel and Nvidia.
If the external monitor also shows lines etc then it is a motherboard problem.
If the external display is OK then it may be a faulty EDP video cable or cable connection (at either end, motherboard or panel) or a faulty panel.
Here’s the [https://download.lenovo.com/consumer/mobiles_pub/legion_y540-15irh_pg0_hmm_201904.pdf|hardware maintenance manual] for the laptop that will help you to check the EDP cable connections and to remove / replace the LCD panel if necessary.
If it appears to be a faulty panel, here’s a link to the [https://pcsupport.lenovo.com/au/en/products/laptops-and-netbooks/legion-series/legion-y540-15irh/parts/search|parts] list.
Type ''Legion Y540-15IRH Laptop (Lenovo)'' in the “Search by product name etc search box”, select your exact model variant that shows up and then click on ''Select Commodity > LCD panels'' to find the part number (FRU) for the panel. Once you have the part number search online using the part number ''only'' to find suppliers. usually the part number is printed on the back of the panel somewhere if you wish to verify that you have the correct one.
Hopefully this will help you to find and fix the problem