when selected index change of dropdownlist event is fired then automatically password field getting empty value

In the on text change event of the textbox write

if(txtPassword.Text != String.Empty)
{
    dropdown.Enabled = true;
}
else
{
    dropdown.Enabled = false;
}

And on the on select change event take the text input of the password text.
When Form post occurred the password text automatically gets empty

Here txtPassword is the id of the textbox and dropdown is the id of drop down.