If the monitor still displays the white line when connected to a different computer, it's likely either the monitor going bad, or the monitor not supporting the refresh rate.

To look up the refresh rate:
- Right-click on the desktop and select "properties"
- On the window the comes up, select the "settings" tab, then press the "advanced" button
- On the next window that comes up, press the "monitor" tab.

Your screen refresh rate should be listed in that window.