When the system powers on and the Debug LED shows the usual POST code sequence but hangs on 2b or you get a beep code as soon as the sequence reaches 2b, then the Mainboard cant detect your Videocard properly.
Check if there is something wrong with the PCIE slot or BUS, the Monitor or monitor cable.
Try cleaning the PCIE slot with an air duster. Turn the Monitor on and off, switch the Monitor cable from one D-SUB or DVI plug to the other in case your Videocard has two.
If that doesnt help try moving to a different PCIE slot. If that doesnt help either try another Videocard on same system if possible.