This problem usually means you email inbox is full. Every email address has a 100 Meg quota. If this quota is exceeded, you will no longer be able to receive any more emails.
The reason an inbox may become full is because of the way your email client (Outlook, Outlook Express, etc) is configured. If your email client is configured to 'Leave a copy of the message on server', a copy of all of your email messages will stay on the server and slowly fill up your inbox. Once the 100 Meg limit has been reached, you won't be able to receive any more emails.
To fix this problem, you'll need to change the configuration settings on your email client to delete the emails from the server once they've been downloaded.
On Outlook do the following:
- Select the menu Tools->Accounts
- Select your account on the dialog and press the Properties button
- Click on the Advanced tab and deselect the 'Leave a copy of the message on server'
