Sonic would be devastated. He and Tails are like brothers. Sonic would likely go into a rage and vow to avenge Tails' death. He would immediately start hunting down whoever or whatever was responsible. Amy would be heartbroken as well. She would probably be the one to try to comfort Sonic while also being deeply sad herself. Knuckles would be angry and ready to fight alongside Sonic to seek justice for Tails.
Sonic would be in shock at first. His best friend and sidekick is gone. He would feel a deep sense of guilt, thinking that he should have been there to protect Tails. This could lead him to become more reckless in his pursuit of the killer. Amy would be crying uncontrollably and would try to organize a proper memorial for Tails. Knuckles would be solemn and would use his strength to protect Tails' legacy. He would also be on high alert, fearing that the same fate could befall other friends.
Sonic would be consumed by grief and anger. He might isolate himself for a while to come to terms with his loss, but then he would emerge with a new determination. He would start gathering information and clues about Tails' death, using his speed to cover large areas. Amy would be in a state of mourning and would try to keep the group together during this difficult time. Knuckles would be seething with rage beneath his tough exterior. He would be ready to use his fists to bring those responsible to justice and would not rest until they were punished.
His friends would be devastated. Scott, for example, might go into a state of extreme anger and blame himself for not being able to protect Stiles. Lydia could be inconsolable, using her banshee powers to try and find some connection to Stiles in the afterlife.
Sonic would probably be very shocked at first. He trusts Amy, so her betrayal would be unexpected. He might initially try to talk to her to understand why she did it.
Maybe Tails was tinkering with Sonic's shoes or some other personal item without permission. Sonic values his stuff and his independence. If Tails was messing around with his things in an attempt to 'improve' them but instead caused a problem, Sonic would likely be angry.
One possible plotline could be that Tails is killed by a powerful enemy during a battle to protect his friends. Maybe a new, extremely strong villain emerges and Tails sacrifices himself to buy Sonic and the others time to escape. Another could be that there is some sort of accident in Tails' workshop while he is working on a dangerous experiment, leading to his untimely death. It could also be that in a dark alternate universe, Tails is targeted and assassinated by an evil organization that wants to disrupt the balance of power in the world he lives in.
Sonic would probably be very confused at first. He might mistake Tails in Amy's body for the real Amy and be really surprised by her different behavior. Knuckles would likely be suspicious and keep asking questions. Cream might be worried about her friends and try to help them figure out what happened. Big the Cat probably wouldn't notice much difference at all, just going about his normal fishing routine.
Sonic's reaction could be very instinctive. He'd be the first to jump into action when he hears about the earthquake. He'd run towards the chaos, ready to face any danger head - on. Tails might be a bit more cautious at first. He'd gather information, maybe use his two - tailed copter to fly over the affected area to assess the damage. Then he'd start using his mechanical skills to create things like communication devices to help the victims communicate with each other in case the regular means are down.
Sonic would be the first to react. He'd be angry and immediately start the search for Tails. Amy would be worried sick and might try to help Sonic in any way she can, like gathering information from the other animals in the area. Knuckles would be on high alert, thinking this could be part of a bigger plan to get to the chaos emeralds. He'd guard the emeralds even more closely while also keeping an eye out for any signs of Eggman's activities related to Tails' kidnapping.
The world would likely be in chaos. Sonic is often the main defender against villains like Dr. Eggman. Without him, Eggman could easily start taking over cities and enslaving the population. His friends would also be in a state of shock and grief, which could temporarily paralyze their ability to act.
It could lead to Tails becoming more independent. If Sonic is constantly getting mad, Tails might decide to do his own thing for a while. This could be a good thing in the long run as Tails could grow and become more confident. However, Sonic might then realize he misses Tails' company and support.
Maybe there was a big misunderstanding or a conflict that drove them apart.