This works only on certain types of car locks and on older-model cars. If you didn't leave a window cracked, use a plastic wedge to pry a window down far enough to get a wire hanger through. Take the wire hanger and straighten it out until it looks like a fishing pole with a hook on the end. Shimmy the hanger down the inside of the window and try to pull up the door lock or push the unlock button.
Again, this may not be possible on all models, but many cars still have an old-fashioned--style lock. You'll need a safety pin or a straight pin, and, if you have access to it, some WD-40 to lubricate the lock. Insert the pin into the key hole and wiggle it around until you hear the lock open. A straightened paper clip works as well.
If you're lucky, you may know someone who has and knows how to use a Slim Jim, which is a thin sheet of metal that looks like a knife blade with a hooked end. Slide the Slim Jim into the space between the car window and the door. This may be covered by a rubber seal that you will need to puncture through. Once the Slim Jim is inserted in the door, feel around inside for the lever that will unlock the door. Once you've found the lever, use the Slim Jim's hooked end to pull it up, thereby unlocking the door. Professional locksmiths use this tool to open car doors; make sure that you or the person helping you out really knows how to use the Slim Jim or the lock could get damaged.
