That is a classic app. Classic apps were shut down on June 30th. Classic apps have been deprecated for quite a while in favor of new apps which have been around for a few years. Emails about this were sent out as far back a February and an official announcement was made at the end of March. In that same time, you should have seen a notice while working with the app in the builder.
Deprecation of Classic Apps begins Monday, June 30th 
- Announcements - Glide Community
At that time you were given a chance to convert your classic app to a new app. If you did not prior to June 30th, all classic apps were automatically converted to new apps. You should have an app in your Glide dashboard with (new) appended to the app name. This should be the new version of your classic app. New apps also use the glide.page domain in place of the glideapp.io domain.
You need to look for the new app that is a conversion of your classic app in the glide dashboard. If you have not worked with that new version in the builder, you may want to review it, make sure it is published, and share the new url with your users. The automatic conversion is not perfect, so you may need to rebuild some of your screens.