There are various reasons why your Kindle Fire HD device may not connect to a Wi-Fi network. But let us not dwell on the reasons for the problem, let us come to the potential solutions:
Kindle Fire HD
Check whether the Airplane mode on your Kindle Fire 10 is active. If so turn it off.
Check whether the Wi-Fi is connected. Do this by going to More > Wireless.
Now, start with restarting your Kindle device.
Use a Wi-Fi analyzer app to check whether the channels are too busy in your area. Click here to download that app.
Then charge your Kindle completely and do a reset. This would bring your Kindle to Factory settings and you can connect once again.