Server Error in '/' Application. Operation is not valid due to the current state of the object

At present I am using SharePoint Services 3.0. I want to upgrade to 2013. First I run the preupgradecheck. It showed one of the error as

Pending upgrade operation detected in this farm.

For that I ran the command psconfig -cmd upgrade -inplace b2b -wait After that all my sites including central administration were down and application pools also stopped.

After number of trails I tried

Windows Internal database service was set as Network Service, changed it to Local and restarted the service - Then ran the command stsadm psconfig -cmd upgrade -inplace b2b -wait -force as per this site

Now everything is fine except for one site. It is not working still and showing error as

Server Error in '/' Application.

Operation is not valid due to the current state of the object. Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.

Exception Details: System.InvalidOperationException: Operation is not valid due to the current state of the object.

Source Error:

An unhandled exception was generated during the execution of the current web request. Information regarding the origin and location of the exception can be identified using the exception stack trace below.

Stack Trace:

[InvalidOperationException: Operation is not valid due to the current state of the object.] Microsoft.SharePoint.Library.SPRequestInternalClass.OpenWebInternal(String bstrUrl, Guid& pguidID, String& pbstrRequestAccessEmail, UInt32& pwebVersion, String&

Please let me know the reason and solution as early as possible.