This is how it solved the problem
<asp:Button ID="btnAddCandidate" runat="server" Text="Submit" CausesValidation="True"
ValidationGroup="1" OnClick="btnAddCandidate_Click" OnClientClick="return validateResume();"/>
function validateResume(){
var hdn = document.getElementById('lnkbtnViewResume');
if(hdn.style.visibility=='hidden')
{
document.getElementById('lblresume').innerHTML="<ul><li>Please Upload Resume</li></ul>";
Page_ClientValidate();
return false;
}
else
{
Page_ClientValidate();
return true;
}
}
Page_ClientValidate()
event calls the page validation event.