You should be able to go to the Store, search for the app and reload it. It should indicate you already own the app - you just need to reinstall it. I did this with numerous apps when I switched app storage to my SD card.
If it was the office 2013 your only choice may be a Restore. Any "APP" on the system should be found in the Store, Try the My Apps option and it will only show what you have or have had on your Surface.