I advise you to contact Shopify's Technical Support team with your inquiry.
I hope you're doing well today!
When you try to enter your old bank details, are you seeing a banner similar to this one?
If so, it's entirely possible that the original bank details entered (and the ones we have on file) are inaccurate, which would understandably make them difficult to replicate.
@Nick_Marketing is absolutely correct - your best next step to resolve this will be to reach out to our support team, who can help you update your bank details on file. To do this, you can visit our Help Center (click here). You'll be asked to log in to your store, enter a topic into the search field, and then select Get support. From here you can choose from any of our currently available contact options, which may include callback, live chat, or email.
Please don't hesitate to let me know if you have any other questions! Take care.