I don't know much about the C style I/O, but my guess was that fgets was reading the '\n' (read from stdin) character into your check string. I performed the following test which removed the '\n' from the "check" string, and it worked.
like i said, i'm unfamiliar with C's input functions, but i would guess there's one that would ignore the '\n' from the user's input...
printf("\nPlease enter a word to search for an instance of:");
check = strtok( input, "\n" );
printf("Couldn't Open \"c:/MyFolder/Data.txt\" ! \n");
printf( "%s\n%s\n", buf, check);
found = strstr(buf,check);
printf( "%s", found );