>I have tried on two different test servers to install SQL 2008 SP3. I get the following:
>
>-----------------------------------------
>TITLE: SQL Server Setup failure.
>------------------------------
>
>SQL Server Setup has encountered the following error:
>
>The NT service 'MSSQLSERVER' could not be started.
>
>Error code 0x84B20001.
>------------------------------------------
>
>When you click the 'OK' button, it says it is getting the sql error, but then it just disappears. When I check, SQL has the updated version number, and if I try to install the SP3 again, it says there are no applicable engines to update. So it seems that the update went part way through, errored out, left no indication of why, and then locked me in so that I cant try again, other than to completely uninstall and re-install.
>
>I can't find anything on MS.
>
>Help please!


Are you able to run SQL Server? What does version say and information about current service pack?

I also suggest to post this question here
http://social.msdn.microsoft.com/Forums/en-US/sqlsetupandupgrade/threads

You most likely will get a better answer if you post there.