Getting error ‘The remote server returned an error: (401) Unauthorized.’ after 30 minutes.

I am using Provide Hosted App & created a web job (Hosted in Azure)which configure my SharePoint Online site collection. This web job uses CSOM. It is working properly but after 30 minute it shows error :

Exception :The remote server returned an error: (401) Unauthorized.
Server         : RD00155D31D5E0
Parent Method Name: CreateAndUploadLogFile
Error Method      : GetResponse
Method Tree       : GetResponse ,Execute ,ExecuteQueryToServer ,ExecuteQuery ,ExecuteQuery ,ExecuteQuery ,CreateAndUploadLogFile ,
Assembly Name     : BizPortal.RemoteProvisioning.Common, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null
Error Type        : System.Net.WebException
Message           : The remote server returned an error: (401) Unauthorized.
Source            : System
Target            : System.Net.WebResponse GetResponse()
Stack Trace       :    at System.Net.HttpWebRequest.GetResponse()
at Microsoft.SharePoint.Client.SPWebRequestExecutor.Execute()
at Microsoft.SharePoint.Client.ClientRequest.ExecuteQueryToServer(ChunkStringBuilder sb)
at Microsoft.SharePoint.Client.ClientRequest.ExecuteQuery()
at Microsoft.SharePoint.Client.ClientRuntimeContext.ExecuteQuery()
at Microsoft.SharePoint.Client.ClientContext.ExecuteQuery()