If an attacking creature is sacrificed after being blocked, can I redeclare the block?
No.  The step for declaring blockers only happens once, followed by the ability to play abilities and effects, which can result in any number of situations including attackers or blockers no longer doing so, being destroyed, buffed, or whatever.  Regardless of the circumstance that results, blockers cannot be changed and the players must deal with the consequences of the units continuing with their combat orders to the best of their ability.