Total Posts: 774
Most likely you have some customization or maybe you have setup scheduled update from your supplier.
It doesn’t looks like hack, since if it was one, there are always some stealing of information or hackers break the database.
You can contact with your hosting and ask them, maybe they restore some back-ups. Also if you know the time when the change was done, you can ask hosting for logs and check what was happening at that time.
In case you edit product and everything goes back to normal, I can offer you to update all the products in bulk using Multi-editor option of Store Manager for Magento application. It will save you from having to manually edit each product. There is 14-days free version of Store Manager for Magento application, so to do the mentioned, there is no need to pay.
Download free version here -