WD 250 GB external hard drive is not being detected by system
First check if your WD is running (spinning), no matter if the blue light is on. Can you hear it close with your ear, or feel vibrations on your hand when it starts.
If it runs, then check/replace the USB cable you are using with another.
Check the Power supply if it gives necessary power.
If you are familliar with disassembling/assembling, then open and remove the HDD from the box and try to connect the disc directly to your PC (using another HDD Data/Power cables).
If it runs, and you can acces it, then something is wrong with the Box.
If it doesn't, then the HDD is broken or damaged.
Please note that if your device is under warranty, DO NOT open the box ! Contact your seller for service/replacement instructions.