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Generally a Kindle can be reset by holding down the power button (or sliding power switch being held in the 'active' position) for at least 20 or 30 seconds.
This tends to (speaking generally about 'all' Kindle models) wipe all data of yours from the Kindle, and leave it a blank slate in factory-released condition. Notes and annotations of yours on the device would be lost unless you copied them off elsewhere first. Books would all be erased and you would have to download fresh copies from the site e.g. Amazon.com where you bought them.
As for the problem. You almost certainly have one key on the keyboard that is sticking, and/or has a defective key-switch, and/or has gotten dirt into it. Crumbs, fine salty and/or oily crumbs, that have gotten into a keyboard or a keyswitch, are particularly baneful and damaging. Just cleaning cannot always cure the problem and the keyswitch or the whole keyboard (as a matter of economies, preferable in terms or labor cost) usually has to be replaced.
Let me know if you need to know more... by the way: A reset will NOT cure the problem, if it is indeed a defective or dirtied keyswitch, or a defective keyboard. And it might have some perils, with an older Kindle. It might even become inoperative with internet connections, for subtle reasons, after a full reset.
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