the installation of Office Server Service Pack 2 (incl. Office SharePoint Server 2007, Project Server 2007, Form Server 2007, Search Server 2008 and Search Server 2008 Express) has a side effect.
The installation of SP2 properly activate the licence expiration date. To resolve the issue you need to reenter the licence information on the "Central Administration / Operation / Convert License Type".
What does it means for your farm?
A trial version runs 180 days before you need to add the licence key. An Enterprise farm with properly activate the licence expiration date will run 180 day. In case you did not reenter the licence key in the 180 days limit (expiration date) then the SharePoint farm is down/inaccessible for users. The content in the farm is not effected.
Additionally we are trying to fix this problem in the next release builds of Service Pack2.
Windows SharePoint Services 3.0 SP2 is not affected on the problem.
Original link to product group post: Link
Manual Workaround: KB 971620
[UPDATE] - Hotfix available for SP2 Side effect: read more...