I found a number of users with this same bug on Stack Exchange: http://elementaryos.stackexchange.com/questions/7680/wingpanel-not-responding-loki (unlike the original poster, I did not install Dropbox—only major install was the Google Chrome repo and app)
I returned from system sleep to find the menu bar gone. When I Ctrl+Alt+Del, I get a notification that wingpanel is not responding, but no feedback if I opt for force quitting it. New Loki install, Intel integrated graphics on Gigabyte GA-H110M-A motherboard.
"sudo killall wingpanel" has no immediate change, but shows a small, white "wiping"-style movement in the top right, like when a notification closes. Sometimes the menu bar restores, but none of the icons or Applications menu respond. (Once I saw the bottom of the message bubble-style popout come out of "Applications", with the triangle part of the bubble pointing to Applications, but all but the bottom white margin of the bubble off the top of the screen. Eventually, I get another notice about how wingpanel is not responding. Force quitting starts the same process anew. I'm unable to get a working menu bar even after rebooting.
Update: The menu bar does become functional again, but only after a notification that I am connected to wired ethernet (even though I haven't touched the computer or installed anything to prompt the change). It quickly becomes unresponsive again.
Hey, unfortunatly I can't reproduce this.
If you or somebody have time to test some stuff it would be great.
It would be helpful to know if it is a indicator or wingpanel.
You could remove all indicators with:
sudo apt-get remove wingpanel-
and test if it is better. If yes add one indicator at a time and figure out which is it.
To get all back just run:
sudo apt-get install wingpanel-