Why I get access denied to a web application in SharePoint 2013?

I have a SharePoint web application in 2013 farm with SharePoint 2010 experience. It was created using host header and classic authentication. 

contents gets pushed in through another authoring farm. 

This web application was running fine and working but suddenly today no users can log in to the web applicaiton. 

site collection administrators, farm administrators no one can log in. 

Everyone is getting the access denied error. 

I am suprised to see this. 

I have another web application in the same server which does not use host headers that runs fine for all the users. 

But this one is suddenly giving this problem. 

I have one another web application in the farm with the same port created using host headers..

this is the ULS logs error

Access Denied. Exception: Access is denied. (Exception from HRESULT: 0x80070005 (E_ACCESSDENIED)), StackTrace:  
 at Microsoft.SharePoint.Library.SPRequestInternalClass.GetAclForScope(String bstrWebUrl, Guid guidScopeId, Boolean fRequirePermissionCheck, Object& pvarAcl, UInt64& lAnonymousMask)    
 at Microsoft.SharePoint.Library.SPRequest.GetAclForScope(String bstrWebUrl, Guid guidScopeId, Boolean fRequirePermissionCheck, Object& pvarAcl, UInt64& lAnonymousMask).

any idea anyone please.....

kukdai