Hi Andrew.
GA uses a different method to track conversions. AdWords uses a conversion tag.
It's likely that your conversion tracking tag was removed from your receipt or thank you page.
You should look there first.
If you're not using AdWords conversion tracking code, you may be importing GA conversions. Go to the conversions section and make sure they're not disabled.
You can also use a great little Chrome extension called "Google Tag Assistant" to help diagnose the problem.