When a busy period goes wrong.
IF THE SITE WENT DOWN
Check whether it was resource limits, which cPanel > Resource Usage will show. If so, caching or a plan upgrade is the fix.
Tell customers plainly what happened and extend the offer if you can. People are forgiving about a shop being overwhelmed; they are less forgiving about silence.
IF ORDERS WERE TAKEN FOR STOCK YOU DO NOT HAVE
Contact those customers immediately, before they contact you. Offer a refund or a wait, and let them choose.
Do not go quiet hoping to fulfil late. That turns an apology into a complaint.
IF PAYMENTS SUCCEEDED BUT ORDERS DID NOT RECORD
Check the gateway dashboard against WooCommerce orders. Reconcile manually, and fix the webhook before the next promotion.
IF THE OFFER LOST MONEY
Work out what actually happened: the discount, the delivery cost, or the volume of returns. Then decide what you would do differently rather than concluding promotions do not work.
AFTERWARDS
Write down what went wrong while it is fresh. The note is what prevents the same thing next time.
