Open Campaigns and look under each status for Sales open at or Sales closed at. A campaign set to open sales after its countdown keeps its products off sale until then, and one set to close them keeps them off sale after its end, for good, until you set Buying back to Always or switch the campaign off. The product page says which, where WooCommerce draws its add-to-cart area: Available from a date, or This offer has ended. A page builder that draws a button of its own shows no line, but the shop still refuses the order.
If no campaign says so, the product was taken off sale by something else: out of stock, no price, or another plugin. This plugin never puts back a product another plugin has taken off sale.
A campaign saved while WooCommerce was switched off keeps its Buying, Apply to, Selection and Exclude products as they were, because the editor cannot show them then. So a campaign keeps stopping the products it stopped before WooCommerce went off, and no others.
Still stuck? Ask the developers who wrote it. We aim to reply within one business day.
