So your applications folder and address book dissapeared..
Question, did you by change sign into or create a new account on your mac?
If you go to system prefers, user accounts. Do you have more then one account showing?
If not, then your applications folder is probably just hidden. To fix it, open your finder / spotlght/ magnifying galss and type in terminal
Then start the terminal app and run this command
sudo chflags nohidden /Applications
Hit enter, then type in your macs admin password, the one you use to boot the mac. It wont sh ow you typing so be careful when you enter it.. Then press en ter.
T hen, see if the applications folder is showing.. If so, try to launch your address book now.. will it open?
If you get stuck at any point let me know where, so I can assist you further.
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Senior Technician and Network Switching Systems Operator / Maintainer.