What does it mean that if one died for all then all were dead?

It means Christ's death for all shows the whole human race was dead. Because one died for all, all were dead, and He died for all so that those who live should live for Him. Those who live spiritually by the new birth should not live for themselves but for the One who died and rose again.

Les explains that the love of Christ constrains us because we conclude that if one died for all, then all were dead—the whole human race. Christ died for all so that those who live, spiritually now as a result of the new birth, should no longer live unto themselves but unto Him who died for them and rose again. Les connects this with Galatians 2:20: “I am crucified with Christ, nevertheless I live, yet not I but Christ liveth in me.” He also ties it to being in Christ: if any person is in Christ, he is a new creature or creation; old things are passed away, and all things become new. Les says this is part of the new birth and salvation, because God has created within us a transformed mind.

2 Corinthians 5:14

For the love of Christ constraineth us; because we thus judge, that if one died for all, then were all dead:

What did Paul conclude from the fact that one died for all?

Paul concluded that if one died for all, then all were spiritually dead. He also concluded that Christ died for all so that those who live should no longer live for themselves but for Him who died for them and rose again.