was there a reboot after uninstallation?.

Check for logs in event viewer, you might find some information there.

No need to run any upgrade scripts.But do test in your test environment before upgrading to SP3 in production to make sure applications are compatible with SP3.

If this is Sql Server 2005, there will be default trace running.You might find more information there.

no, it will not upgrade SSIS Packages.

you have to upgrade ssis packages,recreate all SQL Agent jobs,setup db mail,rebuild index and update stas with full scan,recreate alerts and warnings,enable all startup proc`s and run dbcc checkdb on...

After inplace upgrade..you need to change compatibility mode to 100 manually.

Redgate SQL Compare

were you able to ping the server?

my guess is you did an inplace upgrade...can you tell what steps have you performed after the upgrade?.

Have you done rebuild index and update stats?

For inplace upgrade to SQL 2008, your SQL 2000 must be at SP4 or above.

check the job history.In case any of the scheduled jobs failed during this power failure and you can run them again.

Rudy..Did the unattended sp3 install worked ?

Also if you have any SSIS packages which use 32bit drivers for connectivity make sure you have appropriate 64bit drivers.