A device that supports wifi can say it's connected, but it doesn't mean you'll be able to browse the Internet, or use applications that require Internet connectivity.
A device could be connected to the router, in which case the device will say connected but the router may not be connected to the Internet. The first thing I would try is restarting your router and modem. Be sure to leave them off for a good few seconds when turning off.
If this doesn't fix the problem, another thing I would try is restarting your Nextbook. Or perhaps disconnecting and reconnecting to your wifi. Something I have tried myself in the past that's worked on Windows OS, is forgetting the network completely, and then reconnecting. But note that this means you'll have to type in any details to connect to your wifi such as the password.