Black lines appears on the bottom of my iMac desktop screen

What model iMac are you referring to? If it is a 2021 or later 24" iMac then the display has suffered a hardware failure and needs to be replaced. There are a large number of these models that have had similar issues, to-date Apple has not formally responded which means if you want it fixed it will be an out-of-pocket expense in the $600 range. One thing you can try though if its a 2021 or later 24" iMac is using an external display and simply put it in front of the of the damaged display.

Sounds like you might have a hardware problem. Try plugging your computer into an external monitor to see if that works. If it does, you might need a new screen.