It's midweek and you just found out that your primary candidate is not taking the job offer, he was supposed to start on Monday, what do you do next?

Life happens and things just might not align, the candidate could easily have a justified and logical reason. My first step would be to reach out and find out what I can. If the reason involves benefits, pay, schedule, etc., I may be able to re-negotiate with the hiring manager and see if I can still bring them on board with a modified job offer.

Sometimes candidates just don't pan out. I am a strong believer of having a backup plan. This is what I would do, first, call the candidate and ask why they did not accept, then, I would see if I can re-negotiate. As this person is reconsidering, I am also reaching out to our hiring manager to ask if they will consider our second runner up for the role. If they are willing to offer the role to the runner up, I would send them a job offer.