Make sure that in your config.php file you have $admin_file = "admin";
Make sure that in admin.php file you have define('ADMIN_FILE', true);
If this is an upgrade, make sure that you only have one set of NukeSentinel icons in the admin panel.

If the link that can't be found is .php instead of admin.php, then you have not completed all the edits.

Using NukeSentinel(tm) Control Panel:
Login to your nuke Administration Menu and select NukeSentinel(tm). Over towards the upper right of the panel you should see a Search for: block. Type in the IP and select GO. This will pull up that IP in every table that it finds it in. You want the Blocked IP's table. Click on the X to the far right. At the Are you sure you want to delete the IP prompt, click Delete IP. This will remove the IP from the blocked IP table and your .htaccess file if you are also writing the blocked IP's to .htaccess. Note: Your .htaccess file must have the permissions set to READ/WRITE (666 or 777) to perform this removal. If your Search doesn't yield any results, then either you mistyped the IP or your blocker rules banned it at a higher level. In that case, try retyping the IP, but in the last octet use the wildcard % and try the Search again. Example: Let's say the IP you are trying to unban is 12.112.45.32 (purely fictional for the purpose of this demonstration). If that IP was not found, try 12.112.45.% . If still no results (Highly unlikely) try the next octet wildcard 12.112.%.% .

Manual Method:
Use phpMyAdmin and edit the nsnst_blocked_ips table and delete the IP from the table. If you also write to .htaccess, you will need to delete it from .htaccess. If you do not have access to phpMyAdmin, you can download my NukeToolBox and use it.

NukeSentinel(tm) does a check to see if the IP is "valid" according to IANNA. Since new IP's are being assigned all the time, the IP2COUNTRY table changes often. That is why updates are usually posted every week at http://nukescripts.net.

If applying the updates doesn't resolve the issue then you can add the new IP via NukeSentinel(tm). From the NukeSentinel(tm) admin panel select the Search IP addresses and search for the IP address. When it displays, just edit the range.

NOTE: You must be using NukeSentinel(tm) version 2.4.2 pl5 or later in order for the following to work.
If you are using RavenNuke(tm), there is a setting in the rnconfig.php file that can override the system check altogether. This is not the recommended way but sometimes if you are in a hurry or aren't able to get an updated IP2COUNTRY update, you can use this. Another reason to use this is that IP 127.0.0.1 (localhost) will set this off. So usually on home installations you set this to TRUE.

In rnconfig.php find
$bypassNukeSentinelInvalidIPCheck = FALSE;
and change to
$bypassNukeSentinelInvalidIPCheck = TRUE;

If you are not using RavenNuke(tm), then just add this line to your config.php file:
$bypassNukeSentinelInvalidIPCheck = TRUE;

The likely culprit is that your host does not allow socket connections. GoDaddy is one that does not allow it. To test whether or not that is causing the screen to hang save this code to socket_test.php, ftp it to your site, and call it using http://your-domain.tld/socket_test.php (example: http://www.somewhere.com/socket_test.php)

<?php
//
// This example shows a simple, one-shot HTTP client. It simply connects to a page, submits a HEAD request, echoes the reply, and exits.
//
error_reporting(E_ALL);

echo "<h2>TCP/IP Connection</h2> ";

/* Get the port for the WWW service. */
$service_port = getservbyname('www', 'tcp');

/* Get the IP address for the target host. */
$address = gethostbyname('google.com');

/* Create a TCP/IP socket. */
echo "<u><b>Attempting to create a socket ... <br /></b></u>";
$socket = socket_create(AF_INET, SOCK_STREAM, SOL_TCP);
if ($socket === false) {
echo "socket_create() failed: reason: " . socket_strerror(socket_last_error()) . " <br />";
} else {
echo "OK. <br /><br />";
}

echo "<u><b>Attempting to connect to '$address' on port '$service_port' ...<br /></b></u>";
$result = socket_connect($socket, $address, $service_port);
if ($result === false) {
echo "socket_connect() failed. Reason: ($result) " . socket_strerror(socket_last_error($socket)) . " <br />";
} else {
echo "OK. <br /><br />";
}

$in = "HEAD / HTTP/1.1 ";
$in .= "Host: www.example.com ";
$in .= "Connection: Close ";
$out = '';

echo "<u><b>Sending HTTP HEAD request ... <br /></b></u>";
socket_write($socket, $in, strlen($in));
echo "OK. <br /><br />";

echo "<u><b>Reading response ... <br /></b></u>";
while ($out = socket_read($socket, 2048)) {
echo nl2br($out);
}

echo "<b><u>Closing socket ...<br /></b></u>";
socket_close($socket);
echo "OK. <br />";
?>