Hi,
(1) Did you confirm that there is no active filter ?
This is very common mistake I saw. People add filter and next time forget it and missing entities
Clear all filters:
(2) Try to connect the app service using PowerShell. Confirm under which subscription it is hosted - do it using the same user which you use for the portal. It might be registered a different subscription or your user do not have the permission
(3) Any chance that this is Cloud Services (classic) and not App Services?
The Cloud Services (classic) deployment model was deprecated and since new tutorial speak about app services some people check the wrong portal blade by mistake.
Check if you find your app here: