Hi nitishaa,
Please share the system model and operating system installed. This issue can be caused due to various reasons.
You may try the steps on the link http://dell.to/10F7lZt to resolve common issues with video on Notebook LCD Screens. Please make a note of error code/message if any appears while performing the diagnostics listed on the link. Also, try connecting the laptop to an external monitor and check if the lines appear on it as well or not. This will help to isolate the issue.
Please reply with the information.