Your going to need to install the Motherboard drivers for the new motherboard you got. It will also install the network driver too you can get online.
You must know the model of your mobo, and which chipset it supports soo you can get the proper drivers.
For example if its Nforce mobo , you need to get nforce drivers from nvidia which will include the network ethernet driver. If its VIA chipset then you get Via 4in1 drivers,, and that will install proper ethernet driver.....
Hope this helps a bit! ,,
Your not experiencing a problem, as this is normal after a OS install, Just need the mobo drivers chief!,, gl,