Right-click and click "View Source" on that page and post the complete HTML, as hat contains the actual error.
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html><head><title>Database Error</title>
<meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1">
<meta name="ROBOTS" content="NOINDEX,NOFOLLOW,NOARCHIVE">
<style type="text/css">
<!--
body { background-color: white; color: black; }
#container { width: 400px; }
#message { width: 400px; color: black; background-color: #FFFFCC; }
#bodytitle { font: 13pt/15pt verdana, arial, sans-serif; height: 35px; vertical-align: top; }
.bodytext { font: 8pt/11pt verdana, arial, sans-serif; }
a:link { font: 8pt/11pt verdana, arial, sans-serif; color: red; }
a:visited { font: 8pt/11pt verdana, arial, sans-serif; color: #4e4e4e; }
-->
</style></head><body>
<table id="container" cellpadding="3" cellspacing="5">
<tbody><tr>
<td><img src="databises%20error_files/image.gif" alt="Database Error" height="48" width="48"></td>
<td id="bodytitle" width="100%">Database error</td>
</tr>
<tr>
<td class="bodytext" colspan="2">The database has encountered a problem.</td>
</tr>
<tr>
<td colspan="2"><hr></td>
</tr>
<tr>
<td class="bodytext" colspan="2">
Please try the following:
<ul>
<li>Load the page again by clicking the <a href="#" onclick="window.location = window.location;">Refresh</a> button in your web browser.</li>
<li>Open the <a href="http://www.mydomain.com/">www.mydomain.com</a> home page, then try to open another page.</li>
<li>Click the <a href="javascript:history.back(1)">Back</a> button to try another link.</li>
</ul>
</td>
</tr>
<tr>
<td class="bodytext" colspan="2">The www.mydomain.com forum technical staff have been notified of the error, though you may <a href="mailto:dbmaster@example.com">contact them</a> if the problem persists.</td>
</tr>
<tr>
<td class="bodytext" colspan="2"> <br>We apologise for any inconvenience.</td>
</tr>
</tbody></table>
<!--
Database error in vBulletin :
mysql_connect() [<a href='function.mysql-connect'>function.mysql-connect</a>]: Access denied for user 'my_username'@'localhost' (using password: YES)
/home/my_username/public_html/includes/class_core.php on line 311
MySQL Error :
Error Number :
Request Date : Thursday, May 29th 2008 @ 01:07:15 AM
Error Date : Thursday, May 29th 2008 @ 01:07:15 AM
Script : http://www.maydomain.com/index.php
Referrer :
IP Address : My IP
Username :
Classname : vB_Database
MySQL Version :
-->
</body></html>