I suffered some nerve damage due to an injury some years ago, and its sort of hard to pin down a specific 'feeling' when it comes to it. It starts with being almost numb (that is, no feeling when touched, but a very weird feeling if scratched or pushed.) Again, it's hard to explain. It's a tingle/hot and cold/needle pricks/ache feeling. It seems to happen all at once. Once it was healed completely, those feelings persisted (and still happen today.) Every once in a while, it will prickle or feel suddenly hot/cold. I injured it about 8-9 years ago, and it is still basically completely numb. I hope that helps a little.
When it comes to fingers, they move through the use of muscles located in our forearms. These muscles pull tendons that curl and extend the fingers. In order for a finger to stop operating, it would either need to have tendons severed, which could happen from a puncture, or a knife stab, or something similar. To render those muscles inoperable, that would be nerves that travel down your arm. You can also break the finger, have it heal badly, or have it fused together as the joint shattered in a fall or impact. My index finger healed crooked, but I can still bend it. You might try a break, but make the injury not permanent. That way you don't have to come up with this long explanation, or series of events for why the finger doesn't work. Just make it not work for now, and have it heal over time, a bad sprain, or broken bone could do that, but it would heal and not require extensive medical knowledge to make it seem real.
