<p>I'm running Xorg and my (Qt) program daemonises itself. Now I log out and restart the X server. When I log in again my process is still running fine, but I can't see it.</p>

<p>Is there a way of attatching the new incarnation of the X server to the old process?
If I don't restart the whole server, but log out and in again, is there a way to look at the old process?</p>

<p>Thanks</p>

After the connection to the X server is lost, it is not possible to regain it.
There was an xserver proxy called xmove, but it is quite deprecated and doesn't work with several newer X extension, which are likely used by modern toolkits.
You could try to run your process in another virtual X server like xvnc or (better) NX. NX is a X proxy technology developed by NoMachine. There exist free implementations of NX servers as well.
If you run your program inside such a server, it is possible to attach and detach from it from arbitrary graphical environments.