Chuck Schuldiner, frontman of death metal pioneers Death, dude died of cancer in 2001 at the age of 34. I never had the chance to see them play live but their records are incredible.
Cliff Burton, Bass player of Metallica, died when the band’s tour bus crashed and he was crushed underneath it. The band lost part of its soul and it took them a very long time to get over Cliffs death. Seeing videos of him playing live makes one understand what was lost, as he was an incredible musician.
On a more general note every artist that died very young, many made a big impact to their genre or even to a wider audience without it ever knowing.