Please first check out our status page to make sure everything is operational from our end. The app almost never has downtime (our uptime averages 99.998%), but this could be the issue.
If everything is working as expected, then please ensure that you're not signed into two different shops -- Product Customizer may be trying to access options for a different shop.
If this isn't the case, we'd be more than happy to help out! You can open a ticket to let us know which shop you're having trouble accessing.