Hi , problem with ur db connection , check ur connection string source,catlog,username and password,if web application and sql server in same machine, then use localhost or ip address or simply single dot, Check ur sql instance name also
e:g
Data Source=192.168.0.1\SQLEXPRESS
Data Source=(localhost)\SQLEXPRESS
Data Source=.\SQLEXPRESS
Check sql server browser service running or not , use port to access from other pc if web and sql different machine,port must u check with sqlservermanager.
Port : Data Source=192.168.0.1\SQLEXPRESS,1443
Regards,
Aravind