Comment 3 for bug 965422

I am having a similar experience on a very recent install of 12.04 amd64. This exact series of events have happened twice for me, on two different days:

I log in, and leave the computer for a while (screen locks during this time). Come back; unlock screen and notice the update-manager icon appears in dash showing number of packages requiring update along with blue triangle indicating it wants attention. Clicking on the icon produces no response. Running "update-manager" from terminal also produces no response. The update-manager is clearly in the list of running processes (ps -e | grep update-manager), but is presumably not receiving the signal to open or not displaying.

Maybe the problem is an interaction with the locked screen?

problem can be awkwardly worked around from terminal by:
killall update-manager
update-manager