Dell Inspiron N5110 powering on but not booting up after motherboard change.

I recently changed the motherboard on my N5110 as it wasn't booting up at all and was completely unresponsive. Since I substituted the old motherboard for the new one, it is now powering on but remains on a blank screen and only powers on when plugged into the charger. Also, the laptop beeps three times when powering on which seems to indicate a problem with the motherboard/system board even though it has just been replaced. 

Does anyone have any fix to this? Many thanks.