Mail flow stopped working

Hi , I have installed exchange 2013 server for my client. one worst day i changed sonicwall firewall (im not sure about the reason) exchange 2013 mail flow suddenly stopped .. no incoming and outcoming..its like a silence of hell . i made somechanges in mail flow settings..now im clueless.. im going back to windows 2008 R2 and exchange 2010 sp3 . from windows 2012 R2 and exchn2013 sp1. can u suggest me somthing ..i have few days to confirm my plan to my manager .. Jeya kumar T jeyakumar@outlook.com