Fix Square WooCommerce Payment Failure

My WordPress site was taking credit card payments through Square with no problems, and it suddenly stopped. I didn’t make any major changes to the site that I know of.
I need an experienced WordPress/WooCommerce developer to find the cause and fix it. This is probably a small problem, like a plugin update conflict, an expired Square connection.
Scope:
• Find out why card payments are failing
• Fix it and confirm checkout works with a test or small real transaction
• Write a short note explaining what was wrong and what you changed
Terms:
• Fixed price. Your bid is the full price for diagnosis and repair.
• If the problem turns out to be bigger, stop and tell me before doing anything extra.
• Back up the site before making any changes.
• I’ll give you a temporary admin login, which I’ll delete when the job is done.
• Payment is released as one milestone once I confirm payments are working.
To be considered, start your bid with the word “SQUARE” and tell me:
1. What you think the most likely cause is
2. Whether you’ve fixed Square/WooCommerce payment problems before
3. How soon you can finish
Copy-paste bids that don’t start with “SQUARE” won’t be considered.