If a booked act cancels, the agency is told immediately and GigSpace starts looking for cover from the acts you've already approved for your venue.
You'll be told what's happening rather than finding out on the night. If a replacement accepts, you'll see the new act on the gig.
If nobody can cover it, the gig is marked unfilled and the agency will be in touch. You're not invoiced for a gig that didn't happen.
The approved backup list is worth keeping current with your agency: it's the difference between cover being found in twenty minutes and a phone call at seven o'clock.
If it's happening tonight and you haven't heard, ring the agency. That's the one situation where a phone call beats everything.