Key problem

Last night I was walking to my car I tried to unlock it with
my key button but it wouldn't unlock so I had to unlock it
from the inside. I didn't get a spare key so that's the only
key that I have. The key works fine it's just the buttons
(lock unlock open truck etc.) Do I have to replace the key
or just get the batteries replace also if I do that will I have
to pay to get the key reprogrammed even if I have a
warranty on the car