If you want to restrict your text box for accepting only numeric value then try this code
Create a script function :-
function enterAmount(eve) {
try {
var val1 = (eve.which) ? eve.which : event.keyCode
if (val1 > 31 && (val1 < 48) || (val1 > 57)) {
return false;
}
}
catch (err) {
alert(err);
}
use this code in your code behind page :-
txtPaidAmount.Attributes.Add("onKeyPress", "javascript:return enterAmount(event);");