Using Page.ClientScript.RegisterClientScriptBlock not working

I am trying to fire a pop up as shown below, but it is not working. Please help

public void btnSubmit_Click(Object o, EventArgs e)
{
    if (checkFileExists(Convert.ToString(fileInfo)))
    {
        Page.ClientScript.RegisterClientScriptBlock(this.GetType(), "Msg", "<script type=\"text/javascript\"  language=\"javascript\">function showMsg(){return confirm(\"This image name already exists, do you want to replace it?\");}</script>", true);
        btnSubmit.OnClientClick = "return showMsg()";
    }
    if (something else)
    {
        // It does whatever is here but never pops the question above
    }
}

and on the button I have

 <asp:Button class="Button" ID="btnSubmit" CausesValidation="True" Text="SUBMIT" runat="server"
             OnClick="btnSubmit_Click"></asp:Button>