Calendar Sync doesn't work anymore

[emon]

Hi,

unfortunately, suddenly my google calendar sync doesn't work anymore. After checking the sync settings of my google account I discovered that the entry "calender synchronisation" is missing. Additionally the default system app "Calendar" seems to be gone.

I already did the following to solve the problem:
1. I turned off the phone, took out the battery and restarted the phone --> nothing changed
2. I emptied, stopped, deactivated and activated the calendar storage in the apps menu. --> nothing changed
3. I removed my google account from my phone and connected it again --> nothing changed

Now I'm close to completely resetting my phone, but still see a tiny chance that you could help me with my problem.
How do I get the entry "Calendar Synchronisation" back to the sync settings of my google account? And how do I get the standard system app "calendar" get reinstalled? Also, I would like to know how all this could've happened.

Thanks already in advance, I'm curious to read your answers!
Norbert

[GalaxyNexusAdmin]

Indeed that is a weird glitch emon! I have not yet had this happen to me. Could anyone in your home or work maybe accidentally deleted it? Maybe you downloaded a rogue app that altered your phone? Did you root your phone and install a custom ROM?

[emon]

I resetted my phone now and the calendar app is there again. Everything is fine now, but just hope that this does not happen again.