It happens because

1. You have an error in some js files. Look in your browsers console for more details.
2. The lightbox js failed to load. Can be due to #1.

To debug : try to send messages after disabling thirdparty plugins (if any).

Did you recently install a third party plugin?

Yes it was becuase of FreiChatX plugin . there is some code I need to put inside /views/default/page/elements/head.php

<!--===========================FreiChatX=======START=========================-->
<!-- For uninstalling ME , first remove/comment all FreiChatX related code i.e below code
Then remove FreiChatX tables frei_session & frei_chat if necessary
The best/recommended way is using the module for installation -->

<?php

if(isset($_SESSION["guid"])==true) {$ses=$_SESSION["guid"]; } else {$ses=0;}
if(!function_exists("freichatx_get_hash")){
function freichatx_get_hash($ses){

if(is_file("/home/public_html/elgg/mod/chat/vendors/freichat/hardcode.php")){

require "/home/public_html/elgg/mod/chat/vendors/freichat/hardcode.php";

$temp_id = $ses . $uid;

return md5($temp_id);

}
else
{
echo "<script>alert('module freichatx says: hardcode.php file not
found!');</script>";
}

return 0;
}
}
?> <script type="text/javascript" language="javascipt"src="http://domain.com/elgg/mod/chat/vendors/freichat/client/main.php?id=<?php echo $ses;?>&xhash=<?php echo freichatx_get_hash($ses); ?>"></script>
<link rel="stylesheet" href="http://domain.com/elgg/mod/chat/vendors/freichat/client/jquery/freichat_themes/freichatcss.php" type="text/css">
<!--===========================FreiChatX=======END=========================-->

This was blocking addwidget javascript also!

I removed this from the head.php .

Now everything is working fine. but the chat plugin how can I implement this ?

Thanks and regards.

Change the language setting for your admin user account at the bottom of the settings page

It works ,

Thanks for your answer