Merlin would be very worried. He would probably stay by Arthur's side day and night, using his magic secretly to try and heal him. He might also search for rare herbs or magical items that could cure Arthur.
Merlin's reaction would be complex. He'd be extremely concerned as Arthur is not only his king but also his friend. He'd start by observing Arthur's symptoms closely. He might recall any similar cases he's heard of in his magical studies. He'd be very protective, not allowing anyone he doesn't trust near Arthur. He'd also be conflicted because he has to use his magic carefully so as not to reveal his secret, yet he needs to do whatever it takes to save Arthur. He might even make risky decisions in his desperation to heal Arthur, like using powerful magic that could attract unwanted attention.
Merlin could be heartbroken. He might retreat into himself and become more withdrawn. After all, he has a deep bond with Arthur, and this sudden break - up would be a huge shock.
Merlin might be initially shocked and heartbroken. He has been so close to Arthur, and this sudden forgetfulness would be a huge blow to him.
In some versions of this type of fanfiction, Arthur's reaction is complex. He might be stunned into silence when he first finds out. His mind would be racing with thoughts about all the times Merlin has used his magic to save him or Camelot without his knowledge. After the shock subsides, he may have a heart - to - heart with Merlin. They would talk about the implications of the secret, and Arthur would likely start to view Merlin in a whole new light, as not just his servant but as a powerful ally with a great burden.
He might also get angry. Merlin has his own pride and after all he has done for Arthur, being insulted like this would make him furious. He could start yelling back at Arthur, listing all the times he has saved Arthur's life or the kingdom.
He might be initially startled. Arthur is used to a certain propriety in the court, so having Merlin suddenly on his lap would catch him off - guard. But then, depending on their relationship at that point, he could either laugh it off if they are in a very friendly stage or he might be a bit flustered and ask Merlin to get up if there are other people around.
Merlin is usually very empathetic. He'll listen to the reader's problems without judgment. In some fanfictions, he might share his own secrets and fears to make the reader feel less alone. He'll also do practical things, like bringing the reader food or making sure they have a warm place to stay if they're in a difficult situation. He'll try to show the reader that there are always options and that life is worth living.
Merlin might have used his knowledge of the castle's secret passages. He could lead Arthur through those passages, away from the assassins. Also, Merlin could have called upon some magical creatures he's befriended, like the dragon, to help him. The dragon could fly in and either scare the assassins or carry Arthur to safety.
Arthur might be initially shocked and confused. He's used to Merlin being in control. But then he might find it quite hilarious and start teasing Merlin about it.
In Merlin fanfiction, Merlin calls Arthur sire as a sign of respect. In the Arthurian legend and related fictional settings, sire is a term used to address a nobleman or a person of higher status. Merlin, being Arthur's servant and advisor in many versions, uses this term to show his loyalty and deference to Arthur's position as king or prince.
He could be very surprised at first. Maybe he's just out on a normal day in the forest and suddenly comes across a baby. His immediate reaction might be to look around for any sign of the baby's parents.