You can either:
Go to your last quicksave by pressing and holding F9 (Hopefully you created one)
Switch to another craft
Waggle your mouse everywhere until you're lucky enough to hit "Revert" or exit the save
The last choice is to kill the game through task manager.
I suggest you keep a quicksave handy once you launch any craft, just quicksave right when you spawn on the runway. Or, you can keep a stock craft parked nearby for easy switching.