This doesn't happen always, but happens with a lot of frequency.
Whenever my game disconnects due to connectivity issues, when I log back in, a few seconds later my item quickbar disappears.
I try to click the hotkey "\" to make it appear again but it doesn't work (because in theory is still on, looking at the configurations).
The only way it seems to fix is to go into the game configurations and turn off and on the quickbar.
That resets the configuration and solves the issue.