css not dropping down

I have a css menu on my page (www.matrixitcomputers.co.uk/indexwithflash.shtml) which has stopped working properly.
now the menu doesn't drop down when i hover over it.

Could someone point out what needs to change so this will work?

Thanks

#masthead {
	background:url() repeat-x;
	color: #fff;
	background-color:#999;
	padding: 0px;
	margin: 0;
	height: 73px; /* may need to change this */
	border-bottom: 1px solid #fff;
		
}

#masthead .content {
	padding: 0px;
}

/****************   CSS MENU ************************/


#masthead .chromestyle{
width: 100%;
font-weight: bold;
font-size: .90em;
height: 80px;


}

#masthead .chromestyle:after{ /*Add margin between menu and rest of content in Firefox*/
content: "."; 
display: block; 
height: 0; 
clear: both; 
visibility: hidden;
}

#masthead .telno {float:left;
					width:200px;
					color:#FFF;
					text-align:left;
					margin-left:15px;}

#masthead .chromestyle ul{
border: px solid #BBB;
width: 100%;
background: url(img/menu/nav-bg.gif) center center repeat-x; /*THEME CHANGE HERE*/
padding: 28px 0;
margin: 0;
text-align: right; /*set value to "left", "center", or "right"*/
}

#masthead .chromestyle ul li{
display: inline;
}

#masthead .chromestyle ul li a{
color: #fff;
padding: 4px 8px;
margin: 0;
text-decoration: none;
border-right: 0px solid #DADADA;
}

#masthead .chromestyle ul li a:hover{
background: url(img/menu/bg_over.png) center center repeat-x; /*THEME CHANGE HERE*/
padding-bottom: 10px;


}

/*#masthead .chromestyle ul li a[rel]:after{ 
content: " v";
content: " " url(downimage.gif); /*uncomment this line to use an image instead
}*/


/* ######### Style for Drop Down Menu ######### */

#masthead .dropmenudiv{

position:absolute;
top: 0;
border: 1px solid #BBB; /*THEME CHANGE HERE*/
border-bottom-width: 0;
font-family: Arial, Helvetica, sans-serif;
font-size:12px;
font-weight: normal;
line-height:18px;
z-index:100;
background-color:#333;
width: 200px;
visibility: hidden;
/*filter: progid:DXImageTransform.Microsoft.Shadow(color=#CACACA,direction=135,strength=4);*/ }


#masthead .dropmenudiv a{

width: auto;
display: block;
text-indent: 3px;
border-bottom: 1px solid #BBB; /*THEME CHANGE HERE*/
padding: 2px 0;
text-decoration: none;
font-weight: bold;
color: #fff;
}


* html .dropmenudiv a{ /*IE only hack*/
width: 100%;
}

#masthead .dropmenudiv a:hover, a:active{ /*THEME CHANGE HERE*/
background-color: #000;
color: #6699ff
}

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