disable all the elements in html

I don't know why you would need that but this will work:

// this will disable all input elements
var elems = document.getElementsByTagName('input');
var len = elems.length;

for (var i = 0; i < len; i++) {
    elems[i].disabled = true;
}

Try this,

function disableForm(theform) {
        if (document.all || document.getElementById) {
            for (i = 0; i < theform.length; i++) {
            var formElement = theform.elements[i];
                if (true) {
                    formElement.disabled = true;
                }
            }
        }
    }

Or else you can try this too, as RaYell said

function disableForm() {
    var inputs = document.getElementsByTagName("input");
    for (var i = 0; i < inputs.length; i++) {
        inputs[i].disabled = true;
    }
    var selects = document.getElementsByTagName("select");
    for (var i = 0; i < selects.length; i++) {
        selects[i].disabled = true;
    }
    var textareas = document.getElementsByTagName("textarea");
    for (var i = 0; i < textareas.length; i++) {
        textareas[i].disabled = true;
    }
    var buttons = document.getElementsByTagName("button");
    for (var i = 0; i < buttons.length; i++) {
        buttons[i].disabled = true;
    }
}

To disable the whole page you can find some info here,

The easiest way is to put all form elements you want to disable inside a <fieldset> and then disable the fieldset itself.

An example: http://jsfiddle.net/xdkf9b8j/1/

If you don't want the border around the fieldset, remove it per css.

I suggest to do it the "Lightbox"-style way.

Add an absolute positioned, transparent, full screen div Layer above the Page. This way, the user can't even click on a Link.

To give the user a visual feedback that the page is disabled, you can make the div e. g. 50% transparent black.

BTW, here is also a jQuery Plugin that uses a similar technique.