What happens when you show up to work and no one's there and you can't get in the building?

Perhaps the manager had a family emergency. If the store phone has an answering machine, call and leave a message. Obviously you cannot work if the store is locked. You should call periodically so that you can return to your regular shift at their earliest convenience. You will not be penalized for not working when the store is closed. Your manager should let you know when you can go back to work. If you do not hear something in the next day or two, you may want to consider applying for a job somewhere else.