Try to restart the device by holding down the power button for 40 seconds until the device turns off, then turn it back on. Also make sure Wireless Connectivity is turned on in Settings.
If this doesn't work your tablet may be running outdated system software. Instructions on how to update the Fire HD 7 (4th Generation) can be found here.
If you are still having issues the problem may be with your actual WiFi. In this case check the internet connection of another device, if that device can't connect either try turning your router off and back on again.