Windows server 2012 & MS-SQL Express 2012

Question

I have a web application running 100% for almost 3 years, under windows Server 2008 R2 64bit using MS-SQL Express 2008 R2 and .NET Framework 4.0 IIS 7.5

we decided to move on with Windows server 2012 R2 64bits and MS-SQL Express 2012
but when try to run the application, it fails throwing the error message

"object reference not set"

 

I search the net for answer to the error message and all I found is relalted to MS-SQL Express 2012, I've done as suggested but I still get the same

error "Object Reference Not Set"

 

any expert idea.

 

TIA