You can simply do this with javascript
I assume you have several checkboxes on your form and you want to check whether atleast one checkbox is checked or not on button click
add the following code in your source. just remember 'myform' is the id of the form of your page
<input type="checkbox" name="check_list" value="1" />check 1<br />
<input type="checkbox" name="check_list" value="2" />check 2<br />
<input type="checkbox" name="check_list" value="3" />check 3<br />
<input type="checkbox" name="check_list" value="4" />check 4<br />
<input type="checkbox" name="check_list" value="5" />check 5<br />
<asp:Button ID="Button1" runat="server" Text="Button" OnClientClick="ValidateChecks(document.myform.check_list)"/>
and add the following function of javascript
function ValidateChecks(chk)
{
var flag=0;
for (i = 0; i < chk.length; i++)
{
if(chk[i].checked==true)
{
flag=1;
}
}
if(flag==0)
{
alert("You Did not select any checkbox.");
}
}
I hope you find this helpful