how to repair my blank screen?

The connector may have worked loose or the screen could be dead. It's also possible that the output have been redirected to an external monitor.

Connect an external monitor.
Redirrect the output to the regular screen.

If the connector is loose, you need to open the case and reconnect the connector. In this case, the display options will not show the main monitor.

If the screen is dead, you need to replace the screen.
In this case, the main monitor should be visible but not useable and may display some fault message.

Electro
February 2014