My U2412M monitor stuck in power save mode without display cable.

Monitor shows to power save mode when choose input source as a display port.

Unplunged monitor's all cable (include power code) then pluged in only power code with select input source as a displayport make to power save mode (without displayport cable)

how can i fix my mornitor?

ps. how can i register my mornitor in dell.com. I purchased at Amazon.com (seller is amazon.llc)