Kindle Fire HD is very amazing but software problems can arise in even best device. So if you have tried everything on your Kindle Fire HD device to resolve any software problem but still you are facing problem then you can try to resolve it by hard resetting your Kindle Fire HD.
It should be noticed that once you perfome hard reset on your device, it will lost all the data stored in it. So hard reset should be tried only if this is the last option.
You can hard reset your Kindle Fire HD by following below mentioned steps.
1. First move to the Settings app.
2. Now tap on My Account.
3. Now scroll down to Deregister and tap on it.
4. Now follow on screen instructions and confirm about Deregister.
5. Now move to Settings app –> Device.
6. Now scroll down to Reset to Factory Defaults and tap on it and your device will reset and reboot.
so by following above mentioned steps, you can easily hard reset your Kindle Fire HD.
Source:
http://www.mlivetech.com/2013/01/05/learn-how-to-hard-reset-kindle-fire-hd/