Both Okonkwo and Nwoye were traumatised by Ikemefuna's death.
The elders had decided to place Ikemefuna in Okonkwo's care and soon forgot about him. Ikemefuna was treated like a member of the family and Okonkwo grew very fond of the boy. He was vivacious and intelligent. Okonkwo liked the idea that Ikemefuna was educating Nwoye in the ways of men.
Okonkwo had always feared that Nwoye would be lazy, so it was a pleasure for him to see how, under Ikemefuna's guidance, Nwoye became more of a man, performing manly tasks and grumbling about the silliness of women. Okonkwo took the two boys everywhere with him, but he had a really soft spot for ikemefuna.
The fact that it was he who had killed Ikemefuna out of fear that he might be deemed a coward, overwhelmed Okonkwo so much that he could not sleep for three days and could not eat for two. He drank palm wine during all that time and his eyes had become bloodshot and he grew weak. He could not stop thinking about the boy. Since Okonkwo was a man of action, and all the work had already been done, he had to find something else to occupy his mind and he sought out Obierika to converse with. Okonkwo also felt that he had become womanly because of his grief and decided to 'man up' and dismiss his pain, which he eventually seemed to have done.
Nwoye and Ikemefuna had become inseparable during the three years that he had been part of their household. Nwoye saw Ikemefuna as a big brother and enjoyed all the stories that he told him. He would follow Ikemefuna everywhere and was taught many things by him. This also gained him his father's admiration and led to Nwoye becoming even more dedicated to Ikemefuna.
On the day Ikemefuna was killed and Nwoye realised that he was dead, it seemed as if something had snapped inside of him. Nwoye would never be the same again and this event would forever influence his thoughts and feelings. It was probably this incident which eventually made Nwoye turn against his tribe and adopt the customs and religion of the colonialists.
The death is absolutely devastating to Okonkwo as it shakes his faith in the traditions and the things that he has built his entire life and existence around. It calls into conflict many things he believes about himself in terms of his manliness and bravery and he also fears that it will bring him trouble as his friend Oierika believes it will be an affront to the earth goddess.
Nwoye had a very powerful connection to Ikemefuna and when he realizes that he has been killed, he begins to seriously question his world and the culture he has grown up in that could sanction or even demand something like that. It very likely plays a role in his decision to leave the traditional gods and join the Christians later in the story.