What has happened here?

And how do I correct it?

It seems that you updated hikashop but that before doing so your edited the view listing_price to customize it. In the new version we changed the name of these two translation strings to PRICE_DISCOUNT_START and PRICE_DISCOUNT_END.

You have two solutions:

1. You can edit the view you cusotmized to change PRICE_BEFORE_DISCOUNT_START to PRICE_DISCOUNT_START and PRICE_BEFORE_DISCOUNT_END to PRICE_DISCOUNT_END.
2. You can also edit your translation file in the configuration under the tab languages and add the lines:
PRICE_BEFORE_DISCOUNT_START=""
PRICE_BEFORE_DISCOUNT_END=""

I really appreciate your quick and simple answers. Thanks again!