As the spell becomes Lightning Bolt, it stops having any targets. If the spell’s controller doesn’t change the spell’s target so it has one, the spell won’t resolve.

The target spell loses all characteristics printed on the card and has only Lightning Bolt’s characteristics. These become the spell’s copiable values. Once it resolves, it’s put into its owner’s graveyard and has its normal characteristics again.

Any additional effects that happen while a spell is resolving based on additional or alternative costs, such as buyback and flashback, won’t happen.

Lightning Bolt is an instant with mana cost and the ability “Lightning Bolt deals 3 damage to any target.”