accessing session data

I am new to ROR and I am trying to retrieve data based on a session
value I store. This is a basic app that collects user data and puts
it in the DB. I then store the last insert id to a session variable
on the server. This much works. The problem I am having is if that
user comes back to the site I dont want to show them another form I
want to show them the data they entered and I want to access the data
in the DB via the ID I store. I have played with a bunch of different
things but no go. Anyone have any ideas?

I get the error :
You have a nil object when you didn't expect it!
The error occurred while evaluating nil.order_number

I am trying to do this in my controller (the else condition works
fine)

def show

if session[:id]
   @something = SOMETHING.find(session[:id])
else
   @something = SOMETHING.find(params[:id])
end

end

My show.rhtml is simply :

<% for column in SOMETHING.content_columns %>
<p>
  <b><%= column.human_name %>:</b> <%=h @something.send(column.name)
%>
</p>
<% end %>