Could be corrupt preference list - com.apple.systemuiserver.plist
You can locate this file by
Finder>Go>Go to Folder
copy and paste:
delete this file or move it to the desktop if you want to return it. Can be deleted later off the desktop.
Reboot or in terminal, copy and paste:
return, this will rebuild the plist you deleted.
Third party stat apps (menulets in upper right) might need to be replaced the way you like.
Something might have changed in ML.
Rebuild LaunchServices Database
YOu can run this line of code copy and paste
/System/Library/Frameworks/CoreServices.framework/Versions/A/Frameworks/LaunchSe rvices.framework/Versions/A/Support/lsregister -kill -r -domain local -domain system -domain user
Return, It takes nearly a 60-80 secs to complete be patient it will put you back out at the prompt,
It is possible in Mountain Lion a SafeBoot may rebuild the Launch Service Database if the above is not effective.
There is always a problem with this editor inserting breaks where they are not suppose to be, syntax is important. Let me post it again:
/System/Library/Frameworks/CoreServices.framework/Versions/A/Frameworks/LaunchServices.framework/Versions/A/Support/lsregister -kill -r -domain local -domain system -domain user
I had this issue suddenly (my OS randomly reported "too many open files" in stderr while idling, so I rebooted.. then I lost my icons forever)
All the cache wipes, LaunchServices/plist deletes, and reboots in the world didn't help. This was isolated to a single existing user profile, and creating a new profile was not worth the effort. I just went blind on icons for a while. (No Wi-Fi signal strength was one of the hardest to deal with)
But when I installed the OS X 10.10.3 update, they came back! Fortunately, the updater restored things that shouldn't have gone missing in the first place.