Is it still sounding off?
If it's the factory alarm, using the original key, try to start the car. It won't, but it will at least shut it up.
Close the doors and lock it using the key from the outside. Unlock it using the key from the outside.
I've got an aftermarket alarm in my MR2 and had something similar happen to me.
A friend I let drive my car locked the doors using the power doorlock button and closing the door afterwards.
To my surprise, I had no idea that most cars come with a factory alarm that will still engage if you go along doing what my friend did.
My doors would not unlock and a different sounding alarm went off. I thought my new alarm was acting weird, but I called up the installer and was told about the factory alarm (ECU) being active and was told how to disable it.
Give that a try.