Is there any error code stated?
quote:
Repair Windows Live Messenger
1. In the notification area on the taskbar (near the clock), click the Messenger icon, and then click Exit.
2. On the taskbar, click Start, and then click Control Panel.
3. Double-click Add or Remove Programs.
4. Under Currently installed programs, click Windows Live Messenger, and then click Change.
5. Click Repair.
Try that.