How I've handled it in the past (three times in 20 odd years) is - contact another artist you krow will do a good job, has the date free, who will be prepared to do it for your agreed fee, and have them on standby.
Contact booker and let them know asap. Offer the alternate you have lined up, giving them contact details, ask them to let you or your sub know their decision as soon a possible.
I have found that this approach is welcomed, whether or not they use your deputy, they appreciate that you are being responsible, and are not leaving them high and dry. Never any bad feelings, indeed atist and organiser's relationshp is usually stengthend.
love, john.