Items in the Dock are only pointers to the actual item. One of two things are wrong. Either the items the Dock points to are gone, or the Dock is corrupt.
Check to see if the items you expect the ? marks to launch are actually gone. If one of them is Mail, look in the Applications folder to see if it's still there. If not, that's why it's showing a question mark in the Dock. If Mail
is there, then the Dock preferences are corrupt. You could drag the botched entries back into the Dock, but eventually, something else then disappears that you know shouldn't. It's best to replace the preference file with a new one. Go to the Preferences folder in your user account and delete the file:
com.apple.dock.plist
The Dock will reappear as it did when you first got your Mac. You'll have to reset it to your preferences.
