.classname is working only the first Load

Hi,
 I have an asp.net Application.In this application have a Textbox and it will allow only the Numbers Only.If i press the alphabets, It will display a warn icon
inside textbox.It is working only the first time.If I have changed the dropdownlist and then press the alphabets,,it wont show any Warnicon in IE 8 & IE11.
But it will work in Mozilla in Both sceneario.
I am using .classname for this.
How Can I fix it.
Thanks & Regards,
Dileep