Why was the HMS Hood not in the battle?
The Bismark destroyed the Hood by then.
HMS Hood had blown up after suffering a devastating salvo from Bismarck’s main guns in a previous engagement. Only 3 survivors.
Because she was sunk by Bismark 3 days earlier during the Battle of the Denmark Strait. There were only 3 survivors rescued from the Hood's company of ~1,300 men.
The Bismarck sank the HMS Hood in an exchange of salvos three days earlier near Iceland. One of the Bismarck shells penetrated an ammunition storage area on the Hood, the subsequent massive explosion literally blew the HMS Hood out of the water.
HMS Hood was sunk by gunfired from Bismarck, 3 days before.