I usually try to drag myself in and do the gig if at all possible. If not, I try to find a substitute to offer (clear it with the organizer first - don't just send somebody else). Otherwise if you're really too sick to go on stage, there's nothing to do but cancel and give them as much notice possible. If it's just a verbal agreement (as many of mine are) they can't really make you pay but they might hold it against you for future bookings - i.e. consider you unreliable and not book you anymore. It all depends on your relationship with the organizer, what type of person he or she is, what costs they have incurred, etc.