The best way is the kindle fire utility.
In all likelyhood you were stuck in fastboot mode, it happened to me more than once.
If you have problems, unplug the kindle and turn it off... start KFU and go to the select boot mode screen and choose how you want it to boot up, when you plug the kindle in after this it will start up and KFU should easily detect it and boot it into whatever mode you selected.
I found it's actually very difficult to actually brick the thing.
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