We used to have a list of products with different variations and prices. Now of a sudden the variations disappeared on the product, even though it used to be there. As a result there’s no option of customer to select anymore.
Product variations cannot disappear without a reason, it’s not something that happens in CS-Cart. Have you taken any unusual action recently? Upgrading your store, upgrading add-ons, moving to the different server, tampering with the database?
I don’t think upgrading SEO can do something like that. I’d suggest restoring variations as per @CS-Cart_team instructions first. Then, we can think about what could cause this issue.