connection timeout in asp.net - asp.net

Question

i build a web application and every thing works fine . but after i put it on real work i got a problem
after about 30 minutes working in it any connection will be loading and then through an exception :
request time out.
so i have to restart the server or restart the iis site to access the code again ?
why is that happened and how to fix it.
I use this connection class to connect to the database in the whole code :
public class Connection
{
public static SqlConnection Open()
{
try
{
string ConStr = #"Data Source=.\SQLEXPRESS;AttachDbFilename=|DataDirectory|\SARC.mdf;Integrated Security=True; Max Pool Size=100; User Instance=True";
SqlConnection con = new SqlConnection(ConStr);
con.Open();
return con;
}
catch (SqlException odbcEx)
{
return null;
}
}
public static SqlConnection Open(string ConStr)
{
try
{
SqlConnection con = new SqlConnection(ConStr);
con.Open();
return con;
}
catch (SqlException odbcEx)
{
return null;
}
}
public static void Close(SqlConnection ConStr)
{
try
{
ConStr.Close();
}
catch (SqlException odbcEx)
{
}
}
public static SqlDataReader Query(string query, bool isreader)
{
try
{
SqlConnection con = Open();
SqlCommand com = new SqlCommand(query, con);
SqlDataReader rd = com.ExecuteReader();
return rd;
}
catch (SqlException odbcEx)
{
return null;
}
}
private static SqlConnection con;
public static int Query(string query)
{
try
{
con = (con == null ? new SqlConnection() : con);
if (con.State != System.Data.ConnectionState.Open)
con = Open();
SqlCommand com = new SqlCommand(query, con);
int result = com.ExecuteNonQuery();
con.Close();
return result;
}
catch (SqlException odbcEx)
{
return 0;
}
}
}
how can i fix this problem ??? plz help