What is stopping me from Logging in?

You have a lot going on here, I would try to break it down into pieces, but here is what I suspect is your problem.

SqlCommand cmd = new SqlCommand("select INST_ID, EmailAddress from Tablepass", con);

Your SQL command selects INST_ID and EmailAddress from TablePass. You then execute the ExecuteNonScalar, which when using a Select doesn't return the count of records, it returns the first value from the first row, so whatever INST_ID is for the first row it returns.

What I think you want to do is something more like this:

SqlCommand cmd = new SqlCommand("select count(*) from Tablepass WHERE emailAddress = #emailID AND password = @password", con);
cmd.Parameters.AddWithValue(@emailID, TextboxEA);
cmd.Parameters.AddWithValue(@password, TextboxPW);   

bool goodLogIn = (int)cmd.ExecuteNonQuery() == 1;

This is what I did.

protected void Page_Load(object sender, EventArgs e)
    {
        if (IsPostBack)
        {
            SqlConnection con = new SqlConnection(System.Configuration.ConfigurationManager.ConnectionStrings["HotConnectionString"].ConnectionString);
            con.Open();
            string cmdStr = "Select count(*) from Tablepass where EmailAddress='" + TextBoxEA.Text + "'";
 
            SqlCommand userExist = new SqlCommand(cmdStr, con);
            SqlCommand cmd = new SqlCommand("select INST_ID, EmailAddress from Tablepass", con);
            int temp = Convert.ToInt32(userExist.ExecuteScalar().ToString());
            if (temp == 0)
            {
                ScriptManager.RegisterStartupScript(this, this.GetType(), "script", "alert('User Name Does Not Exist!!! You Must Fill Out Registration First!!!');", true);
                TextBoxEA.Text = string.Empty;
            }
            else if (temp == 1)
            {
                ScriptManager.RegisterStartupScript(this, this.GetType(), "script", "alert('Invalid UserName/Password!!!');", true);
                
            }
        }
    }
    protected void Button1_Click(object sender, EventArgs e)
    {
 
        SqlConnection con = new SqlConnection(System.Configuration.ConfigurationManager.ConnectionStrings["HotConnectionString"].ConnectionString);
        con.Open();
 
        if (true)
        {
            SqlCommand level = new SqlCommand("select accessLevel, Password, INST_ID from Tablepass where EmailAddress = @EmailAddress AND Password = @Password", con);
            level.Parameters.Add(new SqlParameter("EmailAddress", TextBoxEA.Text));
            level.Parameters.Add(new SqlParameter("Password", TextBoxPW.Text));
 
            SqlDataReader reader = level.ExecuteReader();
            DataTable dt1 = new DataTable();
            dt1.Load(reader);
 
            foreach (DataRow dr1 in dt1.Rows)
            {
                int returnedLevel = Convert.ToInt32(dr1[0].ToString());
                int inst_id = Convert.ToInt32(dr1[2].ToString());
                Session["inst_id"] = inst_id;
 
                if (returnedLevel == 1)
                {
                    Response.Redirect("FormAPublic.aspx");
                }
                else if (returnedLevel == 2)
                {
                    Response.Redirect("FormCPrivateNon.aspx");
                }
                else if (returnedLevel == 3)
                {
                    Response.Redirect("FormDPrivateFor.aspx");
                }
                else if (returnedLevel == 7)
                {
                    Response.Redirect("CEOPage.aspx");
                }
                else if (returnedLevel == 8)
                {
                    Response.Redirect("DBPage.aspx");
                }
                else if (returnedLevel == 11)
                {
                    Response.Redirect("FormAPublicL.aspx");
                }
                else if (returnedLevel == 21)
                {
                    Response.Redirect("FormCPrivateNonL.aspx");
                }
                else if (returnedLevel == 31)
                {
                    Response.Redirect("FormDPrivateForL.aspx");
                }
                else if (returnedLevel == 0)
                {
                    Response.Redirect("Oops2.aspx");
                }
 
            }
        }
    }
}

I took out the

TextBoxEA.Text = string.Empty;

from the error message and I was able to Login. The error still displays when you enter an invalid username and password. Thanks.