OK normally if you have one working key, you can use that key to program other new keys. Since you saying the only working key you had is gone. you will have to have a locksmith that can cut a key and program it come out to your home.
Or have the vehicle towed to the dealer to have this done.
Here are the normal instructions for programming new keys with at least one functioning key found in your owners manual
To program the new key:
1. Insert the ignition key in the ignition and start the
engine. If the engine will not start see your dealer
for service.
2. After the engine has started, turn the key to OFF,
and remove the key.
3. Insert the key to be programmed and turn it to RUN
within 10 seconds of removing the previous key.
4. The SECURITY message will turn off once the
key has been programmed. It may not be apparent
that the SECURITY message went on due to how
quickly the key is programmed.
5. Repeat Steps 1 through 4 if additional keys are
to be programmed.
If you are ever driving and the SECURITY message
comes on and stays on, you will be able to restart your
engine if you turn it off. Your PASS-Key III system,
however, is not working properly and must be serviced
by your dealer. Your vehicle is not protected by the
PASS-Key III system at this time.
If you lose or damage a PASS-Key III key, see your
dealer or a locksmith who can service PASS-Key III to
have a new key made.
https://my.gm.com/content/dam/gmown...ntiac/montana/2000_pontiac_montana_owners.pdf