Our current car has so called keyless entry feature where the doors can be unlocked simply by pushing a button found in the door handle. Only requirement is to have the key fob with you when doing it. It has been really convenient when you don't have to dig out the keys from the pocket.
Of course such modern wonder requires power. Especially the key fob needs to use a small amount of energy to transmit the identity signal to the car to let it know you are nearby. The energy needed is really low and the small battery that can fit in the fob can last for years. About four years in this case it seems.
When the battery is out getting in to the car gets a bit trickier. Of course the manufacturer has anticipated the possibility of this happening and there is still the good'ol mechanical lock in the door and the fob hides a traditional key inside of it. But due some design decision the lock is hidden beneath a small plastic cover along the door handle. Unfortunately not using that for four years can cause it to get stuck. Let's just say it's a good thing the car came with two of those fobs and other still had some juice left.
The latest generation of cars are starting to get around these kind of issues by enabling your phone to also hook up with the car. The key becomes redundant when you habe your phone basically always with you anyway. And you can even charge that by yourself when needed.
Or maybe it's just easier (and cheaper!) to get those batteries replaced to get the effortless entry back again for the next four years...