Try the folowing Javascript Function :

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function validatefield()
{
var j 0;
var lenvar 0;
var lenEle document.cancel_complaints.elements.length;
for (i 0; i < lenEle; i++)
{
if((document.cancel_complaints.elements[i].type) checkbox )
{
lenvar lenvar + 1;
}
}

for (i 0; i < lenvar; i++)
{
if(lenvar 1)
{
if(document.cancel_complaints.chksel.checked)
{
j j+1;
}
}
else if(document.cancel_complaints.chksel[i].checked)
{
j j+1;
}
}

if(j 0)
{
alert( Please select atleast one checkbox );
return false;
}

else
{
return true;
document.cancel_complaints.submit();
}
}

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