Boot devices like a mouse and keyboard should never be plugged into the monitor. They should always be plugged directly into the computer.

* Was it purchased directly from Dell or from a reseller?
* The monitor USB upstream cable should be reconnected to the computer and monitor to verify that it is fully seated on both ends.
* Exactly how is the U2412M connected to the computer? Using what ports and what cabling?
* What specific computer (laptop or desktop)?
* What specific video card? What ports are on the video card?

IF I can't use the mouse and keyboard by the usb ports of monitor, any other way the usb ports could be used?

-it is purchased from a reseller, computer valley.

-I had reconnected them many times, problem still not be solved.

-I connected the monitor to the desktop following the handbook of the monitor. The blue VGA cable.

-The computer is Acer AX1470, info of the hardware is here: http://www.tigerdirect.ca/applications/SearchTools/item-details.asp?EdpNo=1188410

* I tried to do something today. I connected the monitor to other desktop and it still not worked. When I connected it to my laptop, the laptop could recognize the monitor (from "signle monitor" to "muti-monitor") but the screen of monitor shows nothing, I attached a pic to show how does it look like.

*you may see the monitor can be recognized by the laptop but the screen is totally show nothing even the power lights does not flash.

It must be the problem of monitor, not other reasons.

IF I can't use the mouse and keyboard by the usb ports of monitor, any other way the usb ports could be used?
* Sure. Any USB storage key or external hard disk drive.

* Turn the monitor off
* Disconnect the monitor power cord from both the monitor and the wall power outlet
* Disconnect all other cabling (USB, DP, DVI, VGA) from the monitor
* Press and hold in the monitor power button for 10 seconds
* Reconnect the monitor power cord directly to the wall power outlet, then to the monitor
* Turn the monitor on
* If the image below appears, then the monitor Self-Test has passed

The problem is not always happened, so I have to try your suggestion when it happens.

Thank you for anwering my question

Hello, that problem happened again today, and I did all the steps as you suggested.

It did not help at all.

At the last second step, when I turned the power button, the monitor totally no response.

The screen was black at all, no flash, no images, the light of power button was black also.

There is NO ANY response of it.

Read this.