As with many airlines, easyJet overbook flights. Getting to the airport early will likely mean you're the first on standby lists. Unlike a legacy carrier they will put people who either have status or have paid more than you. That is not the case with easyJet. Turning up early will likely improve your chances. It's unfortunate but that's the best advice I can think of. If it's any consolation getting left behind is actually exceptionally rare. I reserved seats are paid for, a refund will be due.
Good luck with it all. Its not ideal I know.