Most Companies have a security policy in place which forces users to change their network password every 30 or 40 days. If your company also uses Exchange as your e-mail server then you will need to update your iPhone mail settings with this new password as well.
Until you do this, you will not be able to download e-mail from your Exchange server to your iPhone.
To change your password on your iPhone:
1. Tap Settings
2. Tap Mail, Contacts, Calendars,
3. Tap your Exchange Account
4. Tap Account
5. Key in your new password then tap Done
You can now press the Home button or go back to your Mail and synchronize from your Exchange server