Here my code
#include<iostream>
using namespace std;
void main()
{
        char c,str[10000]; int f=0,i=0;
        while (cin.get(c))
        {

                if (c=='!'||c=='?'||c=='.') {f=0; str[i]=c; i++;} else
                if (c>='A'&&c<='Z')
                {
                if (f!=0) {str[i]=c-'A'+'a'; i++;} else {str[i]=c; i++; f++;}
                } else {str[i]=c; i++;}
        }
        cout<<str;

}

Hello;
Pay attention: "Total length of message is not exceeding 10000 symbols."
try with an input containing 10000 symbols.
for eg: if the input contain 10000 H (HH.........HHH )
your programm output:
Hhhh...........hhh­®Àw$üÓÓ¶€|¨ÿ"HÐ)>

the problem is in the function "cout"
So don't stock c in str; just print it.
i hope that's help you.