Warning: this answer is speculative because I haven't hidden any notifications myself!
find ~/ -mmin -10 -type f replacing
-10 with a more appropriate time to specify how far back
find should go (from the current time).
Depending on how active you've been, you'll get a list of files modified in the period you specified.
To find which file is modified by changing settings in
xfce4-notifyd-config, I made a simple change, moving the location of the notification from bottom left to bottom right.
I then ran the
find command given above. This let me identify
~/.config/xfce4/xfconf/xfce-perchannel-xml/xfce4-notifyd.xml as the file in which at least some settings of
xfce4-notifyd-config are stored.
I hope you can find information in that file that may let you undo the "hidden" setting. To comment out stuff in
.xml files use
<!-- to denote the start of a comment and
--> for the end. In other words, first back up
xfce4-notifyd.xml and then edit the original to comment out any line (or content) that seems to be responsible for the "hidden" setting.
As I cautioned at the beginning, this is just a suggestion and not a proven means of reversing the "hidden" setting!
For the record, here's what my
xfce4-notifyd.xml file looks like:
<?xml version="1.0" encoding="UTF-8"?>
<channel name="xfce4-notifyd" version="1.0">
<property name="notify-location" type="uint" value="3"/>
<property name="initial-opacity" type="double" value="0.580000"/>
<property name="expire-timeout" type="int" value="5"/>