accessing invisible control in javascript

You have to use display style to handle it.

You cannot access invisible controls via javascript. Only those controls that are rendered on client site are available to javascript.

If a particular control is marked as Visible=false at server side, then that control is no more present in the HTML of the rendered page on client machine.

Though there is another way to handle such scenarios. If you need to hide a control but also want to access it in Javascript then play with style:display property.

Thus:
to hide the control

// javascript code
document.getElementById("<%=lblToHide.ClientID%>").style.display = "none";

to show the control

//javascript code
document.getElementById("<%=lblToHide.ClientID%>").style.display = "block";