IPad App not working

Really depends, but you've not really given us much information to go on. I'd guess that this particular iPad wasn't on the same wifi as the Sonos is, but without more data, it's just speculation.

The iPad app, iPhone app, and Mac app all worked for over a year or two, in synchrony without any problems of any significance. Then the iPad app stopped working in the last day or so.

There were no storms or power fluctuations. Wifi is behaving normally. No updates were done manually, except to the MAC today when I turned it on, after the iPad app stopped working. The MAC updated without any problems. All other apps on the iPad are behaving normally. Only the iPad Sonos app has a problem. I hope that is enough information?

Is the iPad on wifi? Does it have its own connection to the internet? What version of iOS is it running? Have you powered it down, and waited 2 minutes, then powered it back up? What version of the Sonos app are you trying to run on it?

All Sonos components and wifi components are plugged into recent vintage high quality surge protectors or CyperPower Adaptive Sinewave UPS.

Your symptoms wouldn't correlate with a power surge.

Basically, what you're saying indicates that the iPad in question is connected to a wifi network that is separate than the one that the Sonos is on. That could be an LTE connection from the iPad skipping connecting to your local wifi, or a repeater device in your home not sending the data on, or even an old version of the software/firmware not making a connection.

The iPad is on the same wifi system. All 3 Sonos components have been linked into their own network since the day I got them. I had one Play 1 with a Play 5 to start, then later added another Play 1. The iPad Sonos app did drop the added Play 1 from the listed components with this problem. I have tried to add it back without success more than once. The iPad Sonos is 7.4, joe to date. Version is 37.2.44160. I did power cycle everything as a basic first move. Cannot say I waited two minutes, so will now do it again.
iOS for iPad is 10.3.3 (14G60), up to date.

Two minutes did not help. I gave other data as a housekeeping measure since a power surge is always something to rule out in this area at certain times of the year.

This iPad does not have phone service capability. It works only on wife and has not been out of the house recently. I do not have a repeater. Wifi signal is strong in all areas with a Sonos component.