<style type="text/css">
#warning-message { display: none; }
@media only screen and (orientation:portrait){
#wrapper { display:none; }
#warning-message { display:block; }
}
@media only screen and (orientation:landscape){
#warning-message { display:none; }
}
</style>
....
<div id="wrapper">
<!-- your html for your website -->
</div>
<div id="warning-message">
this website is only viewable in landscape mode
</div>