Smart locks (Schlage, August, Yale, Kwikset) run on batteries. When the batteries die or the firmware glitches, the lock won't respond to your app or code.
Most smart locks have a physical key override. The key cylinder is sometimes hidden behind a decorative cover or rotated to a non-obvious orientation. Check your installation documentation.
Some smart locks have a 9V battery contact point on the bottom for emergency power. Touch a 9V battery to the contacts and the lock briefly powers up enough to enter your code.
If neither override works, calling the manufacturer's support line is the next step. They can sometimes remotely reset the lock if you're authenticated through their app.
Last resort is physical drilling, which destroys the lock. We avoid this when possible.