Application get crashed

Dear Gurus,

My IIS application get crashed and gives following exception.
Please note that every thing was fine till  two days before i do not know what happens now.

Windows 2008 R2 Enterprise Edition 64 Bit
RAM = 08 GB


The description for Event ID 100 from source Enterprise Library Logging cannot be found. Either the component that raises this event is not installed on your local computer or the installation is corrupted. You can install or repair the component on the local computer.
If the event originated on another computer, the display information had to be saved with the event.
The following information was included with the event: 
Timestamp: 18/03/2015 7:05:43 AM
Message: HandlingInstanceID: 97843fa4-8775-4302-943b-a4d646353e6a
An exception of type 'System.NullReferenceException' occurred and was caught.
-----------------------------------------------------------------------------
03/18/2015 14:05:43
Type : System.NullReferenceException, mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089
Message : Object reference not set to an instance of an object.
Source : Test.Infrastructure.Caching
Help link : 
Data : System.Collections.ListDictionaryInternal
TargetSite : System.Data.DataView GetDataView(System.Data.DataTable, System.String, System.String, System.Data.DataViewRowState)
Stack Trace :    at Test.Infrastructure.Caching.EntityLookup.GetDataView(DataTable dt, String rowfilter, String sort, DataViewRowState rowstatefilter)
at Test.Infrastructure.Caching.EntityLookup.GetDataView(String key, String rowfilter, String sort, DataViewRowState rowstatefilter)
at Test.Infrastructure.Caching.Lookups.GetOldBrandLookUpDataView(String rowfilter, String sort, DataViewRowState rowstatefilter) 
   at TESTWeb.Vehicle.FillEnums()
   at TESTWeb.Proposal.FillDropDowns()
   at TESTWeb.Proposal.LoadProposalPage()
   at TESTWeb.Proposal.Page_Load(Object sender, EventArgs e)

Additional Info:
MachineName : Adeel001
TimeStamp : 18/03/2015 7:05:43 AM
FullName : Microsoft.Practices.EnterpriseLibrary.ExceptionHandling, Version=5.0.414.0, Culture=neutral, PublicKeyToken=null
AppDomainName : /LM/W3SVC/1/ROOT/TESTUAT3-1-130711336318772007
ThreadIdentity : 
WindowsIdentity : IIS APPPOOL\ASP.NET v4.0 Classic

Category: General
Priority: 0
EventId: 100
Severity: Error
Title:Enterprise Library Exception Handling
Machine: Adeel001
App Domain: /LM/W3SVC/1/ROOT/TESTUAT3-1-130711336318772007
ProcessId: 3784
Process Name: C:\Windows\SysWOW64\inetsrv\w3wp.exe
Thread Name: 
Win32 ThreadId:4164
Extended Properties: 


Please help to identity the error.

Special Thanks.

Malik Adeel Imtiaz