The end portal blocks going back to the Overworld vanish as is intended when placing down the end crystals to respawn the Ender Dragon. However, if the end crystals are blown up by the initial Wither spawning explosion, or a bed, prior to the dragon being successfully spawned in, then the end portal blocks will break.

If you have no spare end crystals and all of the portal blocks break, then there is no way back to the Overworld unless you die.