That's never good to hear! To help diagnose this issue, I will need to know a few things:
The checkout should be working as intended; however, in most cases, if there is an issue with it loading, it happens at the time of payment when using a third-party payment provider. If you are using a third-party payment provider, and if you are able to replicate the issue when trying to process a payment, then it will likely require reaching out to the payment provider.
Let me know what you find out.