There are the usual reexamination of the install to make sure all cables are connected properly.
Assuming the new motherboard has wifi, it may require a different driver from the MB it replaced. If the MB came with a driver disk, try installing that first. If there is no disk or the new driver didn't help, look on the manufacturer's web site for the driver specific to your MB. Download and install the driver. If that fails, identify the wifi chip on the MB and look to the chip manufacturer for a driver.