You will not be able to get to your SharePoint Site or Central Admin and when you examine the log you may find the error like below. (Your particular version number may be different).
It sounds little scary but, in most cases fix is simple, unless you have something ‘special’ going on.
This would occur when you have a SharePoint farm and you ran the hot-fix package on some but not on all the servers (servers where you have SharePoint installed, including WFEs and App Servers like Index, Search, etc). Or you have installed the updates on all the servers but have not run SharePoint Configuration Wizard on the servers.
So count your marbles, confirm you ran the hot-fix executable on all servers (or if in doubt run it again). Run SharePoint Configuration Wizard on all the servers. You may get prompted to reboot the servers and you can do that as well. I have not seen rebooting creating any issue.
Log Name: Application
Source: Windows SharePoint Services 3
Date: 9/12/2009 11:37:18 PM
Event ID: 5617
Task Category: Topology
The schema version (126.96.36.199) of the database WSS_ContentDbName on DataBaseServerName is not consistent with the expected database schema version (188.8.131.52) on CA or WFE. Connections to this database from this server have been blocked to avoid data loss. Upgrade the web front end or the content database to ensure that these versions match.
<Provider Name="Windows SharePoint Services 3" />
<TimeCreated SystemTime="2009-09-13T03:37:18.000Z" />
<Data>The schema version (184.108.40.206) of the WSS_ContentDbName on DatabaseServerName is not consistent with the expected database schema version (220.127.116.11) on <WFEName>. Connections to this database from this server have been blocked to avoid data loss. Upgrade the web front end or the content database to ensure that these versions match.</Data>