The younger members of the guild who have only heard stories about Natsu would be in awe. They would look up to him and be excited to see his powers in action. Some of the older members who had a closer relationship with him might be a bit angry at first for leaving for so long, but that would quickly turn into relief and celebration. The guild master would likely welcome him back with open arms, seeing Natsu as an important part of the guild's future once again.
His close friends like Lucy would be extremely happy. They might cry tears of joy and immediately hug him. Gray, on the other hand, might act cool at first but then show his true happiness. Erza would probably be the most composed but still show her delight through a warm smile.
Lucy would be over the moon. She might have thought she'd never see him again. She'd want to catch up on everything that has happened in the 10 years. Gray would start off with some sarcastic remarks but then join in the celebration. Erza would immediately start thinking about how Natsu can contribute to the guild's current challenges. And the rest of the guild would be filled with a sense of hope and excitement, as if a long - lost piece of the family has returned.
Natsu would likely be extremely guilty. He might immediately stop whatever he was doing and rush to Lucy's side.
His friends would be very concerned. Gray, for example, might try to physically stop Natsu from drinking more. Erza would likely be more gentle and try to talk to him, to make him understand that getting drunk won't bring Lucy back.
Natsu's friends would have a range of reactions. Lucy would be distraught, her face pale with fear as she kneels beside Natsu. Gray would clench his fists, his eyes filled with rage as he scans the area for the shooter. Erza would be the voice of calm and reason, though her hands would be shaking slightly as she tries to stop the bleeding. Happy would be flying around in a frenzy, screeching in distress. They would all be united in their determination to save Natsu and seek justice for what has happened.
Gray would be extremely angry. He would immediately start looking for clues as to where Natsu was taken. His ice - make magic would be on full display as he might freeze things in frustration while trying to figure out the situation.
Maybe Natsu comes back with new powers. He might have to face old enemies that have grown stronger in his absence. Also, his relationships with his friends could have changed, and he'd have to work on rebuilding those bonds.
Gray would probably be one of the first to want to go after Natsu. He and Natsu have a long - standing rivalry but also a deep friendship. He'd be furious and determined to rescue Natsu. Lucy would be very worried too. She might use her celestial spirit magic to try and find Natsu's location. Erza would be very focused and strategic, rallying the rest of Fairy Tail to come up with a rescue plan.
Natsu might be initially shocked and confused. He's not the type to expect rejection, so he could be at a loss for words at first. For instance, he might just stand there with his mouth open, not believing what he just heard.
Well, in many fanfictions, Natsu might initially be in shock. He's a passionate character, so he could react with anger. He might lash out at Lisanna or the person she cheated with. He could also go into a state of self - doubt, wondering what he did wrong.
She might become angry at first. Natsu is known for her fiery temper. So, she could lash out at the person who betrayed her, confronting them directly and demanding an explanation.
Natsu would likely become much darker. His normally cheerful and carefree attitude would be replaced with a brooding and vengeful one.