Have you seen following error?
Then this post is to solve it!
As per the error your code has not closed the opened SqlConnection properly. For example
SqlConnection conn = new SqlConnection(
doSomething(); /* If some error occurs here — Next line will not get called and it will leave connection open */
SqlConnection conn = new SqlConnection(myConnectionString);
conn.Close(); // This line will get called in any case — success/failure
So, open your solution in Visual Studio and search in entire solution for all open connections and for all the code implement above suggested solution.
Just a note : If you have written Data Access layer code in code behind file then you are in trouble here. You have to do changes at N number of places. If you would have created separate Data Access layer (Technically Class Library) and Method to do DB operation then your task would have been easy enough!
Implement above solutions. You should not see any issues any more.