First, simply trying rebooting the Nexus 7. Also, as the previous poster said, reboot your router, as I've read that worked for some. Then make sure you used the correct password. You may have typed it in wrong, so do it a few times, very carefully. It's very easy to type in wrong when using a new device, because you're not used to the keyboard.
If those things don't work then here are some other ideas having to do with your router: It's also possible your router is using the 5 GHz bandwidth only that that the Nexus 7 isn't compatible with. It can only handle 2.4GHz, so set the router to dual band if so equipped. Also, if the router is set up for only 802.11n, also allow it to use the 802.11g protocol, as some have said this fixed the problem. Others suggested allowing auto-channel select on the router, while others said changing to a specific channel. Then there are those who had some special settings on the router for whatever purpose, and had to change those back to default in order to get it to work with the N7. Who knows? Everyone seems to have their own theory. Suffice it to say that you aren't the first to have this problem, based on reading the forums, but eventually people got things sorted out.
If nothing works, call Google support at 855-836-3987. They might be able to help you resolve it. Or exchange it for a 32GB model, as the 16gb models seem to have this problem. JUST KIDDING! I saw your other post where you were trying to decide which one to get, so thought I'd tease you about it.
Let us know what happens, and good luck!
Sent from my Nexus 7 using Android Central Forums