Warning! Technical information ahead.
The Google Nexus 7 tablets (and other devices using Qualcom chipsets) sometimes / often looses their wifi connection. It looks alright on the tablet with full wifi reception, but nothing works.
This is due to a problem described here: android.stackexchange.com/questions....
Short description: The tablet looses its IP adress because it doesn't answer alive checks from the router.
The solution is to assign a fixed IP adress for the device in your router. The router is most likely @ http://192.168.1.1 or https://192.168.1.1 (SSL). There you can assign a static IP adress to the tablet MAC adress.
This solved the problem for me, at least in my home network.