There are a number of postings about apps not working. This is hardly unexpected. However, what is unexpected is that, mainly after the November update, a number of people are reporting a major issue with not one or two but a host of apps, failing or disappearing altogether. I have seen from other forums this is a widespread problem, with symptoms which are remarkably similar.
Personally I have lost 'core apps' such as People, Photos, Camera, Messaging, Skype, Films and TV, Calculator, Maps, and the Store itself.It is mainly but not only Microsoft's own apps which are affected. I have lost some important third party apps: WiFi Analyser, WinOpener, etc.
I think it is not unreasonable to say my OS is severely damaged. Fortunately I can still use email and browsers.
I have (for the first time) used a Windows Power Shell command to re-install / re-register apps, both via WinFix and directly. It has not worked. The reason why Power Shell cannot re-install / re-register apps, is in every case stated as "Package could not be registered because there was a merge failure." Meaning?
I have a weird Other list at the bottom of All Apps showing (in my case) nine none-functioning apps, all of which are named strangely like this: "@Microsoft.People_xyz" ---- xyz is a very long mainly numeric code. In a few cases these names appear on none-functioning tiles.
None-functioning Groove Music is listed like that, but is referred to as 'Zune Music'!
I have seen advice to uninstall / reinstall apps which misses the point, because you cannot find the app to uninstall it and you cannot launchStore to re-install it. I have tried unsuccessfully to download / re- install some apps like Films and TV from the online Store (which I can launch). But what I get is a message saying 'you will need a new app to open this Microsoft Store' with no explanation and no way to do it. What can this 'new app be'?
I have used SFC / SCANNOW and DISM which report 'there is no problem'. Who are they kidding? I have used FixWin 10 which uses Power Shell and various Troubleshooters. I used four restore points, which did not work.
It really is a mess. As many people are suffering (and more to come?) I do hope Microsoft will issue a fix, soon. One suggestion is to re-install Windows 10 with the Media Creation Tool. I paid a technician (quite a lot) to install it in the first place. It is not an attractive proposition.