Murphy's law, it will go down at the worst possible time.
When is the worst possible time in retail? Christmas, when it will be challenging to get a tech to fix your computer and you will be busy.
Give all your computers a test run now. Make sure it all works.
Also, ensure you have enough computer consumables, e.g. receipt rolls, ink, etc.
Make sure your backups are working. Worst case, your computer is replaceable, but your information is not without a backup. With a good backup, we often need to replace the computer, and you can be back up and running in a day.