My phone was left in a car overnight and now is not working correctly, screen looks funny and I cannot make calls? Will they be able to assist in a store?

If the phone was on, that exceeds the operating temp. Let it warm up to room temperature & see if things improve. If not, go to an Apple store.

If you are living somewhere that's very cold right now, that may be the problem. Let the phone warm up then charge it up. Effects of cold are generally not permanent.

Make an appointment at any Apple store, see what the genius bar says/does.

You don't metion whether the car was hot or cold.