I set user mode +x, now how can I get my old host back?

Once you have set user mode +x, you may not deactivate it during your current IRC session. The only way to get your original host back is to quit and reconnect to IRC. Please note that if you have autohide turned on (default) you need to turn it off via /msg N SET AUTOHIDE off.
This FAQ entry's short URL is http://www.darenet.org/faq/10