Usually, whenever I got a new device, the first thing I do is done all the updates on the system and on the applications I have on the device. I couldn't believe that by doing that on my Surface device, I turned out REMOVED Mail/Calendar app(s) from the system and could NOT get it back until I did something extreme.
Let me explain what happened: when I first used the Store app, I used the [My Apps] menu to get to the "Apps not installed on this PC", and the list was long because I already been using Windows 8 for for more than a year, and I accumulated quite some apps under my account. So I chose another item "Apps installed on XXXX" which XXXX was my home machine. Then I simply clicked everything, thinking that I want apps on my Surface to be exactly the same as the apps that I used at home on my home PC.
All went well until I realized that there's one app having problem updating, and that is Mail/Calendar/... . I was puzzled, thinking, so I tried and tried and update still failed. So I looked at the screen, and saw that there's an option called "Stop Updating" (or something like that). I clicked it, and update stopped. But when I get back to home screen, I found that Mail, Calendar and other related apps were all GONE. I mean GONE GONE. Not unpinned, but GONE.
And the worst part was that I COULD NOT re-download these apps from Store. Ahhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhh!
Ultimately, I found the following way to solve my problem:
Go to [Setting][Change PC Settings][general], and then click [Get Started] on "Refresh your PC without affecting your files". After a while (like almost 20 minutes), I got my missing Mail/Calendar/... apps back, although I had to do some setting all over again, at least I got my missing apps back.