Does your monitor show anything on display but its not recognized in device manager?
If so then download the latest driver from manufactures site(or from the CD that came with the monitor)
Then right click on desktop and choose screen resolution>advanced settings and choose monitor tab,click on properties,and select driver tab and click on update driver.
Choose "Browse my computer for driver software" and in the next page click on "Let me pick from a list....." in the next page click on "have disk" and then go to the folder where the monitor driver is located and choose it,then click next(it may ask you that its not digital signed in,if so then select the option which says install the driver anyway)and you are done.