Hi Virendra,

Run 'Self-Test Feature Check' (STFC) to check the monitor functionality. Follow these steps to run the test.
- Turn off both your computer and the monitor
- Unplug the video cable from the back of the computer. To ensure proper Self-Test operation, remove both Digital (white connector) and the Analog(blue connector) cables from the back of computer.
- Turn on the monitor.
- The floating 'Dell - Self-Test Feature Check' dialog box should appear on-screen (against a black background) if the monitor cannot sense a video signal and is working correctly. (Refer to image below)
   
- Check for the round black patch & horizontal line on the screen again. If they are still visible, it indicates that the monitor has gone bad and needs replacement.
- If monitor display looks good, try the next step

Reset the Monitor back to factory defaults:
- Press button to start (Refer to image below)
  
- Press any of the rest 4 buttons to activate the OSD menu
- Press the corresponding function icon button on the pop-up window of the OSD
- On Screen Display (OSD) system provides a full range of customizable tools to optimize your display
- When the OSD functions are displayed, enter the main menu. And then press '<' or '>' to select the option.
Press 'OK' to enter into the option
- Select 'Recall' option to restore the monitor back to factory defaults (Refer to image below)
  

I am adding you as a friend. If 'Self Test Feature Check' fails, monitor will need replacement. I need to check the warranty status of your monitor. Please reply with your monitor's order number and the account holder's name. Send these details only as a private message.

Do reply with the results. I would be glad to assist.