This again! So annoying. Many have had this happen to us over the last month or so. It seems to be a glitch that happens when we create layers from Excel. They are created properly, even used and edited. And if you go into a map with the layer, it works and you can even bring up the table. But, you can't edit or see the other tabs by opening from content.
There's one official fix: export it as a file geodatabase, and then re-import it. Also, what worked for me, was just not doing anything and it automatically fixed itself. So, depending on your timeline and what you have to do with it, pick your option. Some have also had luck with opening the layer on a different browser and platform. I opened it first with Firefox on a Mac, and after that everything was fine. Don't know if it was coincidence.
Maybe start a support call with ESRI and see if they have anything better to do now? I think they seem to know something is goofy on their server-side.
Very frustrating. Good luck!