What was Achilles's reaction to Patroclus's death?

The brief answer to this is that Achilles's reaction to
Patroclus's death was one of intense anger and an overwhelming desire for revenge.  It
was when Patroclus died that Achilles finally got over being angry about having Briseis
taken from him.  When he heard that Patroclus had died, Achilles stopped "sulking in his
tent" and went out to take revenge for Patroclus's
death.

Some traditions hold that Achilles and Patroclus
were actually lovers and not just good friends.  Whether that is "true" or not,
Patroclus's death got Achilles angry enough for him to come out and fight.  He then
killed Hector and many others of the Trojans.