I went throught this at work yesterday (most likely a different cause tho')
Right click on an empty area of the Start Screen and select "All Apps" in the lower right corner.
If the app is on that screen, right click on it and select "Pin to Start"
If it's not there (or it doesn't work), then you'll have to check in the C:\Program Files\WindowsApps folder.
It's not going to want to let you in there - and I had to take ownership in order to even see what's in mine.
And, once you're in there I can't see any obvious thing to use to launch the app (I think it runs under the WWAHost.exe process - quick description here: What is WWAHost.exe and Why is it Running?
I can't seem to find any easy "fixes" for this. But I renamed the folder in the C:\Program Files\WindowsApps folder (in my case I was playing with the Amazon Kindle folder) and it stopped working (but the Kindle folder remains on my Start Screen). That leads me to believe that renaming the folder might enable you to reinstall the app (DO NOT delete the folder as there is no telling what consequences these experiments will have!!!!)
The usual caveats go here... BACKUP
everything before starting! Set a Restore Point
before starting Make a System Repair disc
before starting - Press "WIN" and "R" to open the Run dialog...type "RECDISC" (without the quotes) and press ENTER
etc, etc, etc
Just to document this I:
- clicked on "Uninstall" (right click on the Kindle app) in the bar at the bottom of the screen
- went to the Store and installed the Kindle app again
It took a long time (20 seconds or so) to install (the first time it was almost immediate)
The app appears to work normally and I still have the renamed Kindle folder in addition to the newly installed one.