Recheck the payment details and billing information where relevant. You can also check available funds or account restrictions with your bank or payment provider; these are possible checks, not a confirmed reason for the decline.
Before retrying, check whether an order confirmation or completed payment already exists. If the payment clearly did not complete and the provider permits it, you can try once more after correcting any details. Avoid repeated attempts; another payment method may be an option if checkout offers one.
If your bank or payment provider declined the payment, contact them for guidance. We may not be able to see why they declined it and cannot override their decision.
If checkout appears to be malfunctioning, contact Shiftworker Nutrition with a description of what happened and any error message. Never send a full card number, CVV, passwords or online banking credentials.