Go to phpMyAdmin
Select the Database, where you have installed your WordPress site.
Select the SQL tab in the top, next to Structure.
Insert the following:
DELETE FROM wp_postmeta WHERE meta_key = '_sale_price';
DELETE FROM wp_postmeta WHERE meta_key = '_sale_price_dates_to';
DELETE FROM wp_postmeta WHERE meta_key = '_sale_price_dates_from';
Wait patiently .. can take a few minutes.
Hope it helps you out. It has been very helpful to me once when one of my clients suddenly had a 0% discount on all of their products. This little thing solved it in the matter of minutes. They had around 240 products, so saved me a lot of time.