ok try to reset your panel then:
And then logout, log back in.
This sort of solved the issue, but not really.
I like to keep the panel on the left side of my screen. When I did the reset it moved the panel to the top of the screen (then when I did the reset a second time it moved the panel to the bottom of the screen). In each instance it did restore the mintmenu. But when I move the panel back to the left side of the screen, mintmenu sometimes disappears and sometimes remains there. However, in all cases, as soon as I log out and log in, if the panel is on the left, mintmenu disappears, never to come back.
Other weird things would happen too like the calendar applet would disappear when I moved the panel back to the left (and then reappear if I deselected the "expand" option for the panel and then reselected it). Also the icon for my backup program would sometimes appear and sometimes not in the notification area (never had any problems with this before 1.18). The calendar problem does not persist after I relogin, it's fine. But the backup program icon in the notification area is erratic.
Oddly, if the panel is on the right side of the screen, mintmenu works fine (it can be added and stays there after login). It appears that top, bottom, and right side are fine.
So there's just something weird with how the panel works when it is on the left side of the screen. But this is a new problem. None of these issues were there before 1.18.
Thanks for any further ideas on this.