I have run into this issue with windows apps and windows store apps becoming unusable from updating my windows 10 1703 version to the fall creators update 1709 two times on my Dell Inspiron laptop. The first time I followed the advice of rolling back to the previous version of windows and found that it made my entire OS unstable and constantly laggy. Then another hardware issue caused me to turn my laptop in for repairs and eventually found myself with a brand new one. Overnight I found that the fall creators update had auto installed on the new one and again, the Microsoft Store app was nowhere to be found. Didn't want to roll back my OS again and have laggy issues anymore so I searched relentlessly for a solution and still haven't found a viable one that worked. After trying to reset it, troubleshoot it, re-register it, and re-cache it; I tried to re-install it by finding the location of the app package. Unsuccessful at it I was given an error that told me that permissions were incorrect.
Here's the fix that worked for me and brought back all my missing apps and the Microsoft Store app as well: I reset the permissions to the Windows App folder back to the Administrators account of my laptop since I'm the only one using it. Find the hidden folder in the Program Files folder of your OS drive by checking the hidden items option in the view tab in the file explorer. Windows App folder becomes visible and from there you can right click the folder to bring up the properties. Select the security tab under properties to bring up the permissions section and click on advanced. If the owner of the folder shows unknown or can't be verified, then there is the reason for the missing apps and the invisible Microsoft Store app.
Upon further investigation, I found that after the Fall Creators Update, Trusted installer had taken ownership of the folder and permissions for the original Administrators account (which was my account) were corrupted and a secondary version was used in its place and my original one gave a warning that that account needed to be deleted because it was corrupt when I selected to edit or view it. I deleted it and changed the ownership from Trusted installer back to my Administrators account with full permissions. Applied changes and closed the properties dialogue box to the Windows App folder and reopened it and selected enable inheritance for the account. Applied changes and closed the properties dialogue box to the Windows App folder and restarted my laptop. When the restart completed, I went through my apps list and all of the missing ones were back up and visible again and the Microsoft Store App icon was also back up. When I double clicked it, it started right up with no issues and was back to normal. Everything was back to normal and my laptop was back to normal working at its fast high speed capability that I loved.
I'm an Animator by profession but a natural problem solver and I hope this has helped solve an issue that seems to have gone unsolvable for everyone else.