sim808 sheild not working with arduino mega 2560.
please someone help.
buad rate is 115200.
working fine with arduino uno.
That's a very nice modem shield 
Before getting your own code to run - load up a simple local echo program to send your typing to the modem and vice versa. That will let you know you have communication to the modem.
You can check the modem connection status, etc
After that - you can start faffing around with a functional sketch, but always keep that echo code nearby so you can see what is expected & returning.