List your current Ruby and Rails versions (i.e. the versions you are upgrading from).
Check to ensure that you have all your gems configured properly and perform a rake gems install to see if they are all installed properly. Keep in mind that in production you are using RAILS_ENV=production rake ...
Start with the simplest things first.
What are you using with Rails, mongrel or passenger?
There are a lot of ways to handle this - I'd start with the patient one.