The only thing I can think may have done it, was delete old stock number from the database because I couldn't find another way of doing it.
That would definitely break updates of stock levels. Am I right in thinking that you're only having problems updating the stock levels?
Presumably you have access to an SQL prompt. If you don't have any uc_product_stock records, you should probably put them back. I think the "stock" tab in the product node edit page will create one if it doesn't exist.