Does excess combat damage carry over to the player if a blocker is destroyed before damage?

When a creature is declared as blocked, it remains blocked for the rest of combat even if the blocking creature leaves the battlefield before damage is dealt. This is a key rule: a creature does not become unblocked simply because its blocker dies (CR 509.1h). However, being blocked means the attacker's damage is assigned to the blocker — and if the blocker is gone, the damage is simply dealt to nothing (or not assigned at all to the player).

Combat damage is assigned during the combat damage step (CR 510.1). An attacker without Trample must assign all its damage to the creatures blocking it. If those blockers are no longer on the battlefield, the damage assignment still cannot redirect to the defending player — the attacker's damage goes nowhere meaningful. No damage reaches the player.

The exception is Trample (CR 702.19b). A creature with Trample only needs to assign lethal damage to each blocker, and any excess can be assigned to the defending player. If the blocker was already destroyed before the damage step, it has 0 toughness remaining in the assignment calculation — but the rules still require that 'lethal damage' be assigned to the blocker first. In practice, with no blocker present, a Trample creature can assign all its damage to the player.

Example: Your 5/5 is blocked by a 2/2. Your opponent casts a spell to destroy your 5/5's blocker before the combat damage step. Your 5/5 is still considered blocked and deals no damage to the opponent. If your 5/5 had Trample, it could assign all 5 damage to the opponent since there is no longer a blocker to assign lethal damage to.

Without Trample, excess combat damage never carries over to a player from a blocked attacker — the creature is simply blocked and its damage is wasted. This is why Trample is such a valuable combat keyword.