Check your email. If a ride is down during your FP time then you'll receive an email letting you now you can use that FP anytime throughout the rest of the day and it'll give you a list of available rides to use it on. If you don't have a smartphone then go to the CM at the ride that's down and they'll let you know what rides you can use it on. It's not every ride in the park that you can use the FP on