How to store datatable in cache to reuse it?

Something along these lines?

public DataTable GetDataTableFromCacheOrDatabase()
{
   DataTable dataTable = HttpContext.Current.Cache["secret key"] as DataTable;
   if(dataTable == null)
   {
       dataTable = GetDataTableFromDatabase();
       HttpContext.Current.Cache["secret key"] = dataTable;
   }
   return dataTable;
}

You're going to need some mechanism to flush the data table from the cache if it changes. For example you could use an SqlCacheDependency.

As requested, to clear the cache an hour after the table is added you would do:

HttpContext.Current.Cache.Insert("secret key", dataTable, null, DateTime.Now.AddHours(1), System.Web.Caching.Cache.NoSlidingExpiration);

try this

 DataTable mytable;
 String key = "key"
 if(HttpContext.Current.Cache[Key] ==null)
 {
        mytable = GetDTFromDB();
        if(mytable.Rows.Count > 0)
            HttpContext.Current.Cache.Insert(strKey, mytable, Nothing, DateTime.Now.AddHours(1), System.Web.Caching.Cache.NoSlidingExpiration);
    else
        mytable = HttpContext.Current.Cache[strKey];
  }

you can use:

HttpContext.Current.Cache["CacheDataTableID"] = datatableInstance;