The Notes app is now gone from my Macbook Air. It used to be in the doc and sync'd with my iPhone and iPad. Earlier today, I saw the icon still there, with a question mark on top of it, and I could not open the app. Now, there is just a question mark. How do I get the Notes App back on my Macbook Air?
The easiest thing to do might be to check in Lauchpad (the silver rocket in the dock or in /Applications ) to search for notes.
If it's still there, it just got moved, so you can search in Spotlight to find it.
If it's really gone, you can restore it using Time Machine or another backup. Worst case, you would have to re-install the OS but that might need some details about how you purchased Mountain Lion (whether it was an upgrade or came with the Mac for instance).
The question mark on the app means that the app cannot be found. Since the Dock should follow to a new location automatically if the app was moved, it usually means that the app has been deleted (if the app was in the Trash, a separate error should show).
Use a backup
If you have a backup such as Time Machine available, attempt to recover the file from there. This should be the easiest method if you have your backup readily available.
Navigate to the
/Applicationsfolder, open Time Machine, go back in time until you find
Notes.app, select it and click Restore.
Use another Mac with the same OS X version
If you have access to another Mac with the exact same OS X version, you should be able to copy the app from that machine and put it in the
/Applicationsfolder on your Mac with the missing app.
Use Pacifist with an OS X installer
If you have access to an OS X installer, whether that be on disc or USB, you can use Pacifist to browse through and obtain the file.
If you don't have access to an installation media, you can download the installer from the Mac App Store.
Reinstall OS X
If all else fails, a reinstallation of OS X will restore all default Apps.