Most of the time when you race, you stick a spare set of wheels in the wheel truck. Otherwise, if you get a flat, you abandon the race and ride back in the wheel truck

As for what happens- I've had a front tire blowout at 20mph+ and the biggest thing is (just like in a car) DON'T PANIC!! Just keep your arms relaxed, stop pedaling, and don't make any sudden moves/braking. You'll be aight...