Sherlock might initially seem unperturbed, which could annoy Moriarty. He'd use his sharp mind to analyze Moriarty's motives and actions. He could also try to engage Moriarty in conversation to gain more information. He might say things that would make Moriarty second - guess his own plans. For example, he could point out flaws in Moriarty's previous schemes, making Moriarty angry or distracted. This would give Sherlock an opportunity to either escape or plan his next move.
Sherlock would be extremely worried. He might go into a state of hyper - focus, using all his skills to find John. He would analyze every little detail, from the place where John was kidnapped to any possible witnesses.
I really like 'A Different Path'. This fanfic takes a unique turn by changing some of the events in the original story. It shows an alternate version of Moriarty's plan and how it affects the other characters. The writing style is engaging, and it keeps you hooked from start to finish.