I had the same problem after I manually updated more than thirty apps directly on the phone (iPhone 6, iOS 9.3.3) and let the device unattended over night – in the morning most of those apps were greyed out.
All the hints given in this thread were helpful, but did not solve my problem: Restarting, shutting down, closing the apps, clearing the app store cache (clicking ten times a tap icon in the Appstore app), logging out from the app store, clearing left overs in the Download folder on the device (using iMazing) but I couldn't even delete the apps (the confirmation dialog appeared but the app didn't). Since I did not want to reinstall the complete device from a backup I kept on trying:
I noticed that iTunes was able to update some of the apps during a synchronization job. It looked like that with each synch job some of the apps were restored (maybe because there were new updates available?). I went on synching repeatedly and now even some of the apps I tried to delete on the device before were now gone: They could be reinstalled with the help of the Appstore app. As I read in other posts the Facebook app was among these apps (but not the only one for me). In the end all the apps were back, updated and working as expected but at the cost of almost a day work.
If you encounter this problem as well I suggest to try the following:
- close all apps (multitasking gesture)
- clear caches (hold the main switch until „shutdown“ appears, release the switch and press home until the „shutdown“ slider disappears and the home screen reappears)
- hard restart (press and hold main switch and home button simultaneously)
- shutdown the device (press main switch and swipe to shutdown)
- clear the Appstore apps cache (click ten times one of the tab icons on the bottom line within the app)
- try to delete and reinstall the app on the device (easiest way if that app doesn't contain personal data or needs credentials you might not remember)
- synch your device with iTunes, maybe there is an update of the app it can install)
- if you still have apps that don't work and you are about to delete apps with personal data and/or credentials: begin with Facebook (this might be woodoo and not the real issue solver, but anyway…)
- keep on synching with iTunes if necessary; have an eye on iTunes during the synch process which apps were updated and check which app icons are still greyed out afterwards.
If that all fails or you don't have time to play around with your device and wait over and over until iTunes finishes synching: Try to replay the backup during the following night – this might be a more convenient way for you and should solve all the issues at ones.
For the future I will update my apps on the device manually or let it be done automatically (switch in the general settings) or still doing it old-school via iTunes.
I hope these summary contains helpful grains of salt...