Hi Jean,
You might be encountering this issue CSCsc86904: WS03 Cannot log back in to SA after timeout
Symptom:
Cisco Unity administrators are unable to access the Cisco Unity System Administrator via Internet Explorer.
Conditions:
Seen occasionally with Unity servers that run on Microsoft Windows Server 2003, typically after an SA session has timed out. Seen with Cisco Unity 4.2(1) and Unity 5.0(1).
If you think this might be your problem, you'll want to download the Eng_Specials_for_Cisco_Unity_4_2_1.zip found here:
http://www.cisco.com/cgi-bin/tablebuild.pl/unity-42
Apply the Engineering Special that has the fix for this bug, as detailed in the readme in the .zip.
Good luck.
Mike