Hundreds of orders go through at zero dollars

It is 10:05 on a Tuesday. Finance posts in the on-call channel: revenue this morning is far below normal, but order volume looks fine. You check:

• Since 09:30, 412 orders have a total of $0.00. A normal morning has 2 or 3.
• 409 of those 412 orders used the promo code SPRING10. Marketing emailed it to newsletter subscribers at 09:00 as 10% off.
• The promotions admin shows that SPRING10 was last edited at 09:27. No code has been deployed since yesterday.
• A thread on a deals forum titled "free stuff here" links to the shop and has 300 replies.
• The warehouse picks the morning's orders for same-day shipping at 13:00.

What do you do in the next 30 minutes, and what do you change afterwards?