If you just got the key blade cut you could unlock the door manually but the cars security system would not allow the car to be started unless the key transponds the correct security code . Car keys typically contain a tiny security chip . When you put the key in the ignition the car sends it a proximity signal and the chip responds with a code. (It doesnt need a battery to do this -just like security tags etc)
Not to be confused with the remote locking and unlocking. Here the key sends the car a vhf radio signal using a battery in the key. You can manage without remote locking if the battery goes flat etc.
If you still have the original key ,specialists such as Timpson can normally read its security chip code and VHF remote locking signals fairly easily and recode the new key to match both. This may be included in the price as they key is not much use without it. Things get more expensive and complicated if you have lost your keys entirely.