Move closer to the access point and make sure there are no obstructions between the Wii U console and the access point.
Check the Internet connection of other devices connected to your wireless router or access point. If other devices do not connect, this indicates an issue with your home network that will need to be resolved prior to your Wii U connecting online.
Check if the security key of the wireless access point is entered correctly.
Try changing the channel on your wireless access point. Please refer to the instruction booklet that came with the access point for information on how to do this.