9.6 upgrade failed

Upgrade form 9.5 sp2 to 9.6, expected it to be routine.

But it failed at 'Preparing 9.5 environment for upgrade'.

Setup log shows Return code 1603 error.

Serializing xml type Setup.Common.xml.Setupxml.

Reboot, it continues and fails at same point.

It appears that 9.5 files have been deleted.

How can this server be saved?

thanks,

Jill